If you are reading this article, chances are you own a 2012 3rd gen Toyota Prius Two, Three or Four (or you have a base model 2012 Plug-in Prius or non-Four model 2012 Prius C). Because if you own a 2012 Toyota Prius Five (or 2012 Prius C Four or 2012 Plug-in Prius Advanced), then your Prius rolled out of the factory in Japan with integrated fog lights. Unlike the 2010-2011 models where aftermarket kits are readily available, the 2012 mid life refresh models require that you order the radiator sub-assembly grille with fog light openings from a 2012 Prius Five or 2012 Prius Plug-in Advanced and an aftermarket OEM-style fog light kit designed for the 2006-2010 Toyota Yaris to make the system work. The total cost for the project is about $150+ and approximately 2-3 hours of your time, depending on your skill level. While this guide is written specifically for the 3rd generation Prius and Plug-in Prius, the wiring diagrams also applies to the base Prius C models.
The good thing is you save on the OEM-style aftermarket fog light kit since the 2012+ Prius models do not have the outer plastic fog light covers that the 2010-2011 models have. Instead of spending $100+ on a set of 2010-2011 Prius fog light kits, you can expect to spend $35-$55 for a Yaris fog light kit designed for the 2006-2010 model, or $60+ if you are interested in projector fog lights with an LED halo ring. The trim the fog light kit is designed for does not matter (e.g. 2 door hatchback, 4 door sedan, 4 door hatchback) as long as the switch is correct (see below under Parts Needed). What you need from this bare-bone fog light kit are the lamps, the switch and the wiring harness and nothing else. You can discard the included Yaris fog light covers. The price reduction comes in handy considering the fact that you’ll need to invest $102+ on the sub-assembly radiator grille to complete this project.
Parts and Tools Needed
- 53102-47020 – Sub-assembly radiator grille ($102.00 from Toyota Parts Zone)
- For 2012 Plug-in Prius Base, Lower radiator sub-assembly grille with silver trim for 2012-2015 Plug-in Prius: 53102-47090B1
- Flathead and Phillips screwdrivers
- Wire crimping/cutting tool
- 1/2″ diameter wire conduit (approximately 4 feet)
- Vinyl electric tape
- Female and male quick connects, for 16awg wire thickness
- For 2012 Prius C One, Two or Three owners who wish to add fog lights, the part numbers for the outer fog light covers are:
- Toyota Yaris 4 Door Sedan Chrome Clear Fog Lights Lamps; any trim will work, 2-door hatchback, 4-door sedan, 4-door hatchback ($35-$60+)*
*Be sure to read my entry about acquiring a fog light kit with the correct switch: 2006-2011 Toyota Yaris fog light switch (Prius-compatible design vs. Prius-incompatible design)
If you are comfortable with taking apart your Prius and working with wires and simply just want the wiring diagrams, you can find it a few paragraphs below or by clicking here to jump directly to the diagrams. Most generic OEM-style aftermarket 2006-2010 Toyota fog light kits are constructed in a very similar fashion, albeit some have different types of plugs, missing plugs, other include a combination of matching plugs and sockets, etc. But fundamentally, they are all the same. And if they are missing plugs (i.e. just open wires), you can simply attach quick connect joints to move forward with the installation.
Understanding the Fog Light Kit’s Wiring Harnesses
There are two main wire harnesses that is included in a standard OEM-style aftermarket fog light kit. One harness is for the fog light switch and this is the harness that has a green plug which plugs into the switch. This green plug has 4 wires coming out of it, usually two yellow wires, one red and one black. They may be in different colors but their function and construction is usually the same. These 4 wires go into a wire conduit and terminates at the other end in a red wire (sometime with plugs, i.e. bullet terminals, male or female connect, etc.), a black wire with a spade terminal, and a yellow wire with a 1-pin female custom plug (usually brown) .
The other wire harness supplies power to the fog lights. This harness has two H11 bulb plugs, each of these two plugs has a white (or red) and black wire coming out of them. The white wire terminates at a 4-point relay. A red wire with a round-ring and a 15 amp inline blade fuse attached also goes into the 4-point relay. Another red wire with a custom plug (i.e. male/female quick connect or bullet terminal) goes into the 4-point relay and finally a black wire with a spade terminal goes into the 4-point relay.
Download the high resolution PDF for this diagram here: 2012 Toyota Prius OEM-style aftermarket wiring diagram (non-Five models)
Let’s start with the switch. There are four wires, usually in the following color and in the following order: YEL, RED, YEL, BLK. The two yellow wires are likely spliced somewhere within the wire conduit. These two yellow wires power up the LED and also supplies a 12V power source that completes the switch when it is pressed. The black wire provides grounding for the LED’s to light up and that’s its only purpose. No ground = no functional LED, but the switch will still work. You just won’t know that it’s on. When the switch is pressed, pin #1 and pin #2 is completed i.e. bridged and energized. That is, the red wire carries the 12V power source from the yellow wire in pin #1 and sends it to the relay. The relay becomes magnetized and then turns on the fog lights.
Where does the yellow wire get its 12V trigger power source from? This depends on how you want to wire up your fog lights. If you want to have them turn on only when either the parking or head lamps are activated, then you tap this wire onto the head light dimmer wire (more on this later). If you want to turn them on whenever you like regardless of the parking or head light state of operation, then you connect this to the ignition or an accessory wire. Avoid connecting it to an always-live 12V source (e.g. directly to the positive terminal of the 12V battery) as the fog light illumination LED will stay lit, even if the car is off. For this do-it-yourself installation guide, we will be getting the 12V power source for the switch from the headlight dimmer wire.
Understanding How the 4-point Relay Work
The relay has four terminals:
- Terminal 1 grounds the relay.
- Terminal 2 receives “trigger” power, usually from a switch.
- Terminal 3 draws 12V constant power. On a conventional vehicle, this simply connects directly to the positive pole on the 12V battery.
- Terminal 5 redirects the 12v constant power and sends it to the fog lights.
Terminals 3 and 5 has a metal tab between them that is open at rest (when it is not energized). When you press the fog lights switch to the ON position, the 12V power is sent to terminal 2 and energizes an internal electromagnet in the form of a copper coil. When this coil is energized, it attracts and pulls down the metal tab, closing terminals 3 and 5 and creating continuity. Power drawn from the 12V constant source from terminal 3 is now sent over to terminal 5 and then distributed to power the fog lights. That’s it in a nutshell. Here’s how the circuit looks like diagram form:
Pre-installation Fog Light System Test – completely optional
Now that you’ve had a crash course on wire harnesses, switches and relays, let’s test out the system before we proceed. You can skip this part but it will only take about 10 minutes of your time and it will allow you to test if your switch, wires and lights work. I think it is better to find out early on in the installation process rather than mid-way into an install job to discover that certain components do not work.
So to test, you need to access a 12V constant power source. You can find this in the under-hood fuse box, located on the right side of the engine bay. It’s a black box. Remove the black box and identify these two vacant 30A fuse housing with a single male blade terminal in each of them (circled in yellow). They sit right below the jump-start terminal:
The upper fuse housing takes a 30A fuse for the head lamp cleaners and is available only on the Prius Five models only. Non-Five models will have a vacant housing as depicted in the photo above. The 12V source directly below the aforementioned housing is not used by any Prius trim models, i.e. it’s a spare 12V source. So you have two 12V source to use.
You can use a battery power testing lead cable wires with alligator clips for quick test connections:
Attach one red alligator clip to one of the two available 12V power source in the under hood fuse box as depicted earlier and connect the other red alligator clip to the red wire from the switch wire harness.
Connect one end of the black alligator clip to the black wire from the switch wire harness and connect the other black alligator clip to a metal surface, i.e. a bolt or fender. At this point, your switch should light up:
Pressing the switch should light up the fog light “ON” indicator LED:
If you have another set of battery testing lead cables with alligator clips, you can test out the entire system. Connect one of the red alligator clip from the second battery testing lead cable set to the remaining open 12V constant source in the under hood fuse box, then connect the other end of the red alligator clip to the red wire from the fog light kit wiring harness that has the 15A inline fuse attached (this wire connects to terminal 3 on the 4-point relay). Be careful and ensure that none of the red alligator clips or any exposed wires that are connected to a 12V source touches any metal surface. Connect the black wire from the 4-point relay (terminal #1) to one end of the black alligator clip and then connect the remaining black alligator clip to a metal surface:
Press the fog light switch to the ON position:
Fog light system passes pre-installation test. Now we can move forward with the installation.
Step 1 – Bumper removal
The first step requires you to remove your bumper. Since I’ve written a comprehensive DIY guide on how to remove the bumper off of a 2012 3rd gen Toyota Prius in another article, you can read it by clicking on the following link: DIY: How to remove the front bumper off a 2012 3rd gen Prius.
Step 2 – Swap out the radiator sub-assembly grille
This step is actually harder than it seems. Once you have removed your bumper, set it on protective shop towel to avoid scratching the bottom of the bumper. I pulled down the rear sets and worked on the bumper in my cargo (protected by the all-weather cargo mat). Turn the bumper around to reveal the OEM fog light mounting holes:
To remove the radiator sub-assembly grille, take a flathead screw driver and dislodge the white clips that secures the two lower tabs from the grille onto the bottom of the bumper:
With the two lower white tabs removed, place them in a safe place and take your flathead screw driver and carefully dislodge the tabs on the grille that secures directly onto the front bumper. I don’t remember the exact number of tabs you need to dislodge, but I think there are about 20. There are tabs on all four sides of the grille. Start by dislodging the top row of tabs, then work the bottom row and finally the sides. You may need a second set of helping hands to help pull the grille as you dislodge the tabs because the grille is tightly secured onto the bumper. And unless you’ve successfully removed an entire row (top or bottom), the grille has the tendency to retract back into place and reattach itself in the process.
With the stock grille removed, you can swap in the 2012 Toyota Prius Five radiator sub-assembly grille with fog light openings. Installation is simple, snap all the tabs into place and reattach the two white clips on the lower grille extension you’ve removed at the beginning.
I recommend pushing in the lower row of tabs first, followed by the upper row and then the sides. Be sure all tabs are aligned and pushed into their corresponding slots on the front bumper.
Step 3 – Mount your fog lights
Mount the fog lights from behind. Simply align 3 of the 4 tabs on each fog lamp with the tab indentation around the fog light openings behind the bumper. One tab will be hanging loose (upper tab away from the center of grille) and this is by design. Secure a self-tapping screw (included with fog light kit) on the lower outer tab of each fog lamp onto the fog light light mounting point on the front bumper and tighten with a Phillips screw driver.
Now we are ready to wire up the fog light wire harness and connect it with the switch wire harness. At this point, you can remount your front bumper onto your Prius. If you do, leave the lower left and right splash shields open so you can connect the fog light wires later.
Step 4 – Install the switch and switch wire harness
This step takes approximately 15-20 minutes of your time and the procedure is relatively simple. You need to remove the switch panel assembly to access the instrument panel junction block (i.e. main body ECU) wire harness and to route and mount the switch wire harness. The below set of instructions for step 4 applies to both the 2012 3rd gen classic Prius and the 2012 Plug-in Prius. It is not applicable for the 2012 Prius C models. However, I like to mention to 2012 Prius C owners that the the fog light switch can be mounted on two locations
- in the switch cluster located on the dash panel to the left of the steering wheel, or
- you can route the wires down the center console to switch cluster where the parking brake sits and install your fog light switch there. Same location where the Eco and EV mode buttons sit.
Start by removing your inner door sills. You can remove it by lifting along the edge, length-wise, that’s facing the cabin, with your fingers and popping the sill off the retainer clips. Pull the entire assembly towards you. Lift up the sill with gentle force.
You’ll need to remove the driver side kick panel next. Start by removing the cap nut by simply twisting it off with your fingers. Then at the locations indicated on the following picture, grab the edge of the kick panel near the clip location and pull outward. Again, do so with gentle force. There are two blue push clips that snaps the kick panel in place. If one or both is stuck on the chassis, you’ll need to pry them off with some needle nose pliers and reinstall them onto the kick panel you’ve just removed.
Finally, remove the entire switch panel assembly to gain access to the instrument panel junction block wire harness. Remove the single screw right below the hood release latch.
Next remove the hood release latch assembly. Do this by lifting the latch (you’ll pop your hood at the same time) and press down on the protruding tab. While pressing down on this tab, slide the entire latch assembly down towards the floor.
With the hood release latch assembly removed, you can remove the entire switch panel. Remove it by grabbing hold of the lower edge and gently pull the entire assembly away and towards the driver seat. The are six clips that hold this panel onto the dashboard.
After detaching the switch panel assembly, unplug two wires behind it – one connects to the mirror switch and the other connects to the dimmer switch. Remove an empty switch panel and replace it with your aftermarket fog light switch and then set the entire panel aside.
Now you have access to the instrument panel junction block wire harness where you will locate and isolate a brown wire from the harness plug and install a T-Tap quick connect on it.
To make the job easier, I removed the plug from its socket. To remove the plug, press down on the retainer clip and pull the retainer down. Pulling the retainer clip will push the plug out of its socket and free it. I isolated the brown wire from the main wire bundle by temporarily undoing the black vinyl tape, isolate the brown wire, and redo the tape:
With complete access to the brown wire, make a small incision with a wire stripper on it. The wire is 18 awg so use the correct size stripper. Make sure you do not cut the wire and only strip 1mm of it to expose the copper underneath the insulation.
Attach a correct size T-Tap (18 awg) onto the incision. If you use a larger T-Tap, the metal contact will not fit snug and jiggle with road vibration. This can cause intermittent open circuit, inadvertently turning your fog lights on and off during operation.
Attach a male quick connect terminal to the end of the yellow wire from the switch wire harness. You may need to cut the original connection to do this:
Plug the yellow wire now with a male quick connect into the T-Tap you’ve attached earlier onto the brown wire in the instrument panel junction block wire harness. For extra safety and security, you may choose to wrap the T-Tap and male quick connect with vinyl tape. If your male quick connect terminal is already insulated, you can skip this step. Ground the black wire from the switch harness. You can attach this to any metal surface area.
I grounded the black wire onto the metal surface directly to the right of the instrument panel junction block wire harness plug. There is a metal screw that is holding another plastic panel in place. If you are going to use this location, make sure the metal spade terminal on the black wire from the fog light switch wire harness makes direct contact with the metal frame. That is, do not mount the black wire onto plastic panel.
You can either route the red wire from the switch wire harness through the channel pictured below in Step 5 and into the foot well area, or route the red 12V trigger wire from the fog light wire harness from the engine bay and into the cabin. Regardless of choice, these two wires will be connected at some point. If this red connector has a custom plug, you may need to snip it off and reattach it later in order to be able to complete step 5.
If you have excessive left over wire from the switch wire harness, you can do one of two things. You can bundle the wire together with a wire tie and zip tie the entire bundle onto a frame behind the switch panel assembly. Or you can shorten the switch wire harness by cutting it down to length and reconnecting the wires with butt connectors. Zip tie the wire harness securely onto an available frame.
Step 5 – Route the 12V power supply wire through the firewall
This step in combination with step 2 (swapping out the radiator sub-assembly grille), I’d say are the hardest part of the entire installation. To route the 12V power wire from the switch through the firewall, you need to puncture a hole in the rubber grommet that secures the main wire harness from the engine bay into the cabin. This grommet is located right behind and above the gas pedal:
An easier way to do this, in my opinion and if you have small or slim hands, is to pop open the hood and look for the same grommet from inside the engine bay. This is what it looks like:
If you look carefully, there is a 1-inch “nipple” that extends out from the perimeter of the main rubber grommet. This “nipple” is entirely hollow with an indentation near the tip. If you can, use a small sharp pair of scissors or garden pruner to snip off the cap. This will open up the passage to which you can route your 12V power supply from the fog light switch harness through. Getting your scissors to this area of the engine is equally challenging but alleviates you from repetitively pushing a sharp object from the floor of your cabin until you puncture a hole through the rubber grommet. Sometime the force will push the entire rubber grommet cap out of its place, meaning you’ll need to push it back in from the engine bay. Punching a hole from inside the cabin sounds easy but I tried and gave up after 20 minutes and went with the engine bay route. Just make sure your cutting tool is heavy duty and very sharp, like a pair of short garden pruners, otherwise you’ll spend unnecessary time twisting and cutting.
As mentioned in step 4, you may need to snip off any plug attached to the red wire from the switch wire harness because it will not fit through this passage on the firewall grommet. The passage is approximately 3mm in diameter. The red wire on my kit has a female bullet connect with clear insulation. I had to remove the insulation to make the bullet fit through the passage. Once in the engine bay, I reattached the insulation. If you snipped off any plug, reattach it with a butt connector.
Once the 12V wire is inside the cabin, pull it down so you have enough slack to work. From the footwell, route the wire to the left and along the stock wire harness anchored to the floor of the footwell. Zip tie the wire to this stock wire harness at three points:
- At the base of the grommet
- Near the middle, just behind the brake pedal
- To the upper left; zip tie it to a hard piece of harness – you can’t miss it.
The last thing you want is to have the wire dangle out of place and being caught in the brake and gas pedal. I also insulated the wire with plastic wire conduit to make it look OEM:
Make sure the wire is secured tightly along the factory wire harness anchored against the footwell flooring.
Route the wire behind the instrument panel junction box and through the inverted triangular channel as pictured. There is a factory wire harness routed inside the same inverted triangle passage:
Once the 12V wire passes through the inverted triangle passage, connect it with the 12V power wire from the fog light switch wire harness. At this point we are done with the cabin side of the installation. Tidy up any loose wires, snip off any excess tie from zip ties, ensure that everything is secured in place and reinstall all the panels that were initially removed.
Step 6 – Install the fog lights wire harness
I think the easiest way to tackle this step is to first identify the mounting points, i.e. the points where you will be zip tying, then layout the entire fog light wire harness along the engine bay aligned to these points. Here is a diagram of where to zip tie for the maximum security:
So essentially, there are five zip tie points. If you look at the engine bay and divide the engine bay in half, half of the zip tie points will lie to the right (i.e. driver side). These points will lie along the hood release cable. On the left half, i.e. the passenger side, you will be zip tying the fog light wire harness along the factory wire harness that is clipped along the radiator support. Here are some pics of the mounting location for the zip ties:
Driver side, near the corner of the inverter, right by the driver side headlight:
In front of the inverter:
Passenger side, near windshield washer fluid reservoir:
Depending on your kit, you may need to be creative with the mounting location of your fog light wire harness ground wire. Here is an example of where I mounted my ground wire:
Check again to ensure that all the zip ties are secure and that the wires are mounted securely along factory harnesses. Ensure that the fog light wire harness are not too close to heat generating source. You can tuck the fog light wire that runs across the passenger side of the engine bay underneath the chassis. Once you are sure that the main fog light wire harness is routed securely across the engine bay, route the driver side fog light plug down the driver side. Route the passenger side fog light plug down the passenger side. The passenger side is a bit more difficult to route because of the windshield wiper reservoir and other wires and harnesses in the way. Reach underneath the left and right splash shield beneath the bumper and connect the plugs into their respective fog lamps.
Take the relay and the 15A inline fuse holder and mount it onto the factory wire harness near the fuse box with a zip tie:
Chances are, the terminating red wire that extend from the red wire with the 15A inline fuse holder either has a custom plug or no plug. If it has a plug, snip it off and attach a female quick connect terminal onto it. Mine had a ring terminal:
Next, take the modified red wire from the 15A inline fuse holder and run it along side the thick white wire and into the under hood fuse box as depicted:
If the wire is too long, you can shorten it by cutting it and the reattaching the shortened wires with end-butt connector:
If you are like me, I wire loom every wire that is visible to the naked eye. Here is the segment of the 12V power wire inside plastic wire conduit within the under hood fuse box:
But I didn’t stop there. I loomed up all the visible wires:
You do not need to go through this extra length for the install but I like to finish my projects off looking as OEM as possible. I think I’ve achieved that. Once you are happy with your wire route and configuration, plug the red wire you’ve routed through the under hood fuse box with the female quick connect terminal into one of the available 12V constant power source:
Reattach the cover onto the under hood fuse box. Now we are ready to test out the newly install fog lights on your 2012 non-Five Toyota Prius.
Step 7 – Test the fog lights
Press the fog light switch to the “ON” position with your head lights OFF. Your fog light should not turn on. Now turn your head light stalk to the parking light position and press your fog light switch to the “ON” position. Your fog light should now illuminate. Turn you head light stalk to “ON” i.e low beam, the fog lights should still remain illuminated. This completes your fog light test procedure base on the aforementioned set of installation instructions.
For illustrative purpose, the fog lamp on the right (driver side) is equipped with a 7.5W LED bulb. The fog lamp on the left is equipped with a stock 55W H11 halogen bulb. Both fog lamps are wrapped with Lamin-X protective yellow films.
The left side with the 55W stock halogen bulb heats up within seconds. The 7.5W LED bulb on the right side remained cool, even after 5 minutes of operation. The right side fog lamp appears greenish because of bluish-white light given off by the LED in combination with the yellow Lamin-X film vs. yellow-ish halogen light on the left against yellow Lamin-X film.
Step 8 – Aim your new retrofitted fog lights for your 2012 Toyota Prius
According to Toyota fog light aiming instructions, visual aim is made with the top of the beam 11.2 inches (284 mm) below the lamp center at 25 feet (7.62 m) away with the lamp facing straight forward. Make sure the ground is flat and level. You can also aim the lights at a distance of 9.84 feet (3 m) away. Instead of 11.2 inches, the top of the beam must be 4.41 inches (112 mm) below the lamp center.
To help illustrate this, I’ve created a diagram on aiming fog lights 25 ft away from a wall: