I didn't know the "pillar" nomenclature either, but a quick jaunt on Google explained it:
Trusa mentioned that in the European "X" model "the wiring is oem from the camera in the backdoor and to the D-pillar," and that in the European "G" model, it "has the oem wiring from the camera to the A-pillar".
Does that mean just the power wiring, or is the wiring that would actually carry the video feed, already there as well?
Getting from the D-pillar to the A-pillar somehow seems less daunting to me than the thought of needing to thread cables through to the back door as well, so it is making me hopeful to think there could be some existing wiring in there already. (Though it would be even nicer if it went all the way to the A-pillar of course!)
I've also read in the meantime that the factory cameras for the RAV4 are 6 volts instead of 12 volts, whereas most aftermarket cameras (like the ones that go into the light bulb spot) are 12 volts. I'm wondering if that's why people installing the 12 volt cameras need to figure out how to, for instance, splice the power to them from the backup lights. Maybe the 6 volt cameras would already have power available.
I'm not sure if that should make me more interested or less interested in trying to use a factory camera. If there were a lot of existing wiring it certainly would tilt towards a factory 6 volt camera.
(I don't know if the European wiring is representative of what's in the USA sold models-- I'm presuming it is, but that's just an assumption.)