I bought a wireless backup camera yesterday and got it installed this afternoon before the rain showed up. I hung the camera inside he rear glass in my HHR and for the transmitter I tapped into the voltage feed to the left backup light. The receiver has power tapped into the red feed wire to the head unit and it's own ground. The camera is high definition and has built in distance lines which I checked with the help of my daughter. The distances are within 6 inches which is not bad. Since I tapped the backup light signal, the camera only turns on and sends a signal and causes the switch to rear video about 2 - 3 seconds after going into reverse.

I bought the unit from a local shop and it is made by these folks: http://www.performance-teknique.com

Sounds great! What model is it that you used? 

I bought the camera which is the little "bullet" camera on the website listed above and the wireless tx/rx were separate items. I guess the wireless can work with the license frame cameras too but I wanted one that I could mount inside the rear hatch of my HHR under the 3rd brake light.

What ever kind you get I would suggest using the video wire to test things and then installing the wireless tx/rx units so you know the camera works okay. I wired mine to the left backup light power supply (as per Chevy electrical diagram) so it does switch modes when in reverse. This means I can not manually switch to the camera as an aux input which I did not want to do.