My head unit has input for a back-up cam, which I installed along with the head unit. It has a reverse sensor so that the screen automatically switches to the cam when the shifter is moved into reverse. I wired this in with the reverse lights since those were near the camera and a logical line to tap into for this effect. The challenge I presented myself with was I wanted to install a switch to turn on the camera at any time, but not light up the reverse lights. When flipping the switch, I wanted the head unit to think I was in reverse and automatically change to that screen. I didn't want to run any more than one wire from the front to the back, so with the help of a couple diodes, I managed to meet this goal. First, the shot of my "schematic." Keep in mind, I don't know the actual symbols, so I used my own.
The power source for the switch is the cigarette lighter in the center console. This goes to the switch panel behind the drink holders (below the climate controls). Into the back-up sensor of the head unit, then down the line to the back of the Rav. The diodes prevent current from going to the reverse lights. The placement of the diodes allows for current to go from the reverse lights to both the camera (to power it) and to the back-up sensor in the head unit. The reverse light has it's own power source (I did not include it in the drawing) which also powers the camera when the vehicle is in reverse. The theory seemed like it would work fine and in fact, it works exactly the way I intended. Now I can be driving forward and be able to switch on the back-up camera to view behind me without the reverse lights coming on, as well as have the camera while in reverse; both scenarios cause the head unit to automatically flip the the camera's view.
A picture showing the wiring job: