"If I purchased a head unit I would purchace a unit that has a rear view camera. The camera would turn on only when the car is in reverse."

I know you want it to come on in reverse, but since I didn't want to spend the bucks on a head unit with video input for a camera, I got one that generates a wifi signal, sends it to an app on my iphone and now I have an easy to install back up camera. Lovin' it! Makes getting out of the mall a breeze!

It's a wifi backup camera that is installed inside my hatch, getting power off of a license plate light:

Car Vehicle Wi Fi Backup Rear View Reversing Camera System for iPhone Android | eBay