In writing my do-it-yourself guide for adding OEM-style aftermarket fog lights onto a 2012 3rd gen Toyota Prius Two, Three or Four (non-Five models), I prepared a diagram in Adobe Illustrator to help weekend do-it-yourselfers understand the wiring component of their fog light kit. The diagram is included below but a comprehensive DIY installation guide to add fog lights to a non-Five model mid cycle refreshed 2012 Toyota Prius will be made available later this week. The guide will include swapping out the radiator sub-assembly grille, mounting the fog lights, installing the switch wire harness, installing the fog light wire harness, testing and aiming. The installation guide is also compatible with the earlier 2010-2011 Toyota Prius models, minus the fog light bezel step which the 2012+ models lack; the wiring instructions will be identical.
The following diagram is designed for use with the 2006-2010 Toyota Yaris OEM-style aftermarket fog light kits (but will work with any aftermarket OEM-style fog light kits designed for the Toyota Yaris, 2010-2012 Prius, Prius C, Highlander, Rav4, Matrix, Camry, Scions, etc.). I am almost certain that Toyota uses the same fog lights (the actual lamps) interchangeably on all their vehicles, including their Lexus brand. If you're tech savvy, you can even create your own wiring harness from scratch using this diagram after sourcing the parts (the switch will be the hardest to source out but you can also use any generic switch).
Download the high resolution PDF for this diagram here: 2012 Toyota Prius OEM-style aftermarket wiring diagram (non-Five models)
This diagram will work with all OEM-style aftermarket fog light kits designed for Toyota vehicles. From my experience installing fog lights, it should work with kits designed for other vehicle makes too including store sold kit like Blazers. It certainly works with kits designed for Hondas and Acuras. The diagram is generic but depicts the fundamental wiring component that comes with most aftermarket kits. Of course there will be variations on the kit, depending on the manufacturer, but this diagram will get you going regardless of presentation. When in doubt, I recommend using a multimeter to measure voltage.