I have a 350Z and there's simply no place to mount a rear head unit on the car unless I wanted to get into some fancy custom body work. The rear license plate is mounted in a rectangular recess. The upper ledge over the plate is where the hatch release and two lights to illuminate the plate are located. It's also too shallow.
So, after watching the video where an angled reflector was demonstrated on the back of an SUV with the head unit pointing straight down, I got an idea of mounting the unit behind the license plate. The plate could be mounted on standoffs to create the necessary space and the reflector would be peeking out from the bottom. This would protect the head unit from the elements and give everything a clean look.
If the time comes when a rear unit is essential, I think this would solve a lot of mounting problems.