Particlewave had a DIY with a bit of review on a rear view mirror system. I think in the end he did something different, but you might want to consider it:
Android rear view mirror with reverse camera/GPS/DVR
I have a wired system in one of my cars I put in years ago. Basically did a 2 DIN stereo install, mounted the camera on a license plate frame and followed other wire bundles with the video output. If I was to do it now, I'd definitely just go with a BT camera.