The camera is powered by any 12V source (not the head unit). I tapped into my reverse lights since they are close to the camera. The camera doesn't turn on unless the car is in reverse. But like the other poster I wish I had wired it to a constant 12V source so that I could turn the camera on whenever i want, like to see what's on my trailer hitch or to see if my car isn't on train tracks at a stop, etc.
I would NOT get this one (http://www.amazon.com/dp/B005Q65JHM?psc=1). I had that and not only did it get moved often, it only lasted through two car washes before the lens got all messed up. I had to trash it and re-install a new one. At the time, that camera was $20.
I use a Crimestopper (http://www.crimestopper.com/products/safetyplus/cameras/sv6818emII.html). I did not pay $100 for it though. I think I paid $40-50. It's been going strong for a couple of years now. It's embedded into a hole you have to drill, but it looks factory and was worth the effort.
The ones that mount to a license plate don't look that great in my opinion. They stick out like a sore thumb and just look like a lazy installation to me. Have pride in your workmanship and make it a goal to look like it came from the factory that way.