My understanding is that an '07 cannot be reprogrammed in the GM Tech system to recognize a backup camera. There is a workaround, but it involves buying and installing an interface from PAC-Audio referred to in this post:
That entire thread contains a lot of useful information on the topic. mmal4 was the trailblazer in successfully getting a camera to work on an '07, but it was not easy. He chose to use the OEM camera, which required a voltage adapter because it's a 6v camera. Aftermarket cameras are generally 12v and would not require an adapter. I installed a SwivelCam aftermarket camera on my '08, but there are a number of good aftermarket cameras available. See www.hitchcam.com for info on the SwivelCam and some other alternatives. I've also heard about a Sanyo camera that removes the "fish-eye" effect that is common to rear-view cameras - I'd give serious consideration to that one too.
I bought at back up cam for $79.00 from Costco for my SLE that works as well as any factory installed cam I have ever had. All you do is fasten the eye for the cam to your license plate and it send a signal to the display on your dash wich you can hard wire in to the fuse panel of plug in to a power point.