Might be worth contacting 4D Tech (Kentucky, I think). Those guys inevitably have a way to hook an aftermarket camera to the factory harness and also have an on/off switch. I've bought several things from them in the past, good to work with. Some things can be pricey, though.

 

Can the auxiliary switches be used to power a reverse camera?

 

My understanding is the camera is on at all times. It is the radio that controls whether you see it or not. Controlled via inputs that identify is backing up.