Instead of installing a navigation system with video input
The app would just need to be able to talk to a wireless camera mounted on the back of your car and be very easy to launch and turn off when needed to back up.
Most of these cameras use 2.4Mhz frequency, so I suppose there is no way for an Android phone to communicate with them, but couldn't one be made with something else like bluetooth? Maybe a dock with a 2.4Mhz adapter?
Seems like a good idea for some company if they can make it easy enough to use when you need it.