I used the Voyager Wireless Camera. It is an always on camera and the monitor sits on my dash. The Voyager works pretty well. It is not as good as a wired camera but a whole lot easier to install. It has always worked while moving slowly (backing up) but sometimes loses connection for a few seconds at highway speed. Works 99.9% of the time.
Voyager WVOS541 four Camera Enabled Digital Wireless Observation System with 5.6" color LCD monitor, connect up to 4 wireless cameras and 1 wired camera, build-in microphone, 960 x 234 Resolution