I think I'd take power from the rear all the way to the front binding a thin wire to the AV signal wire. It's simple and would only be activated when in reverse. However, I shall await your advice!

 

Looked at mine and the advert diagram. dont like the idea of a thin wire running the length .Why not just bring a wire down behind the door rubber to the drivers fuse box.
Pick a fuse that is ignition switched ..pull out a fuse and sick the wire in one hole and push the fuse in . then go buy one of these on e bay to finish the job when you are happy.

 

OK sounds like a plan. Thank you.

 

Damn it the mirror clips don't open quite enough for my auto dim mirror. I expect I'll manage to bodge something though.

 

How did I miss this thread in 2019? This seems amazing.

I truly am wondering if it's possible keep the factory PCB for the automatic headlights function. Without it, my car would display a "headlights are off" warning message all the time. My mirror is still in perfect conditions, but it's only a matter of time.