Hi Peeps,
When I did mine it was quite simple, I used a 4 pin fused 12v relay which takes power directly from the battery only when reverse gear is selected, the 4 pins on the relay are:
Accessory (camera)
Ground (tapped from reverse ground or chassis ground near battery)
12v (battery positive)
Switch (tapped from reverse light)
So yellow RCA to headunit for video then the rest is done at the boot
I just found my old thread here
https://www.e90post.com/forums/showthread.php?t=1668165