If it was me I'd just ad one of those cheap Ebay back up cameras($35 complete with screen). The lens just bolts to the license plate and the one I bought has a screen that you can unplug and put away when you don't want it seen.Even works well at night.
http://www.ebay.com/itm/4-3-TFT-Foldable...=item5673082e2f