I will look into the WiFi hitch camera mentioned above.
For a free hack, try this:
Using 2 iPhones, establish a face-time call. You will have to toggle camera orientation, but I strapped my iPhone to the bumper of my TT with the back-side camera active and pointing away from the TT. I placed my wife's phone with the Face-time call active on the dash of my Expedition.
While the picture is wider than I would like, the picture was sufficient to aid me as I am really poor at backing. I could hear my wife's directions too as she was the spotter. During research, I saw that some of the wireless camera have a 40' range. The Face-time range is unlimited.
Not a great solution, but it is free.