My use is different than your intended use. I borrowed a friends car
that had a built in backup camera and MAN did that make it easy to
align my trailer to the hitch.

I purchased this cheap camera == http://www.ebay.com/itm/230884341173.
I found that this style was better at aligning my trailer to the hitch
than the cameras that mount to the license plate. I mounted it onto a
long strip of aluminum and throw it over the top of the tailgate only
when needed. I use a small 3.5" LCD screen that can handle two
cameras and auto switches between the two. Plug into the cig lighter
and away I go.

My next tow vehicle will definitely have this as standard equipment!

- John