I took apart a cigarette lighter adapter (12v to 5v) and used it's PCB which is small enough to put it inside rear view mirror from where I picked up 12V. The camera supply is 5v and it is close to the mirror, so I connected it by short tin cable to PCB inside...
My car is Exec. and the mirror needs power supply for darken circuit in it. I don;t know if all models are equipped with that sort of mirror.