I had a nightmare mounting mine... I wanted one to use both on the car and when towing the caravan, and as I have also boarded up my rear window, I wanted to use it as a rear view mirror as well.
On the roof, it has to be so far away from the rear edge, because of the curved door top, it can't see the ground for many feet right behind you.
On the top of the back door, wiring becomes harder, plus it still can't see over the spare wheel.
Inside the rear screen, same as on the roof, no close up visibility.
Then because of the spare wheel, you have nowhere else to go, until you get down to numberplate level, but down at that level, all you get is dirt on it, and it can't see over the car behind, so makes it hard to use as a rear view mirror.
In the end, I went with a Waeco system, made a mount to fit the monitor where the rear view mirror is, and as I converted mine to a hard spare wheel cover, I made a mount for the camera, on top of the spare wheel cover. The Weaco has 2 camera's in one. The first looks behind you, and the second, when you put the car in reverse, opens a cover, exposing a second downward wide angled camera, that looks straight down the spare wheel. I can't quite see the tow ball, but I know that it is only about an inch under the wheel. The down side was it took a lot of sorting to make sure the mount was steady, to stop vibration when driving, and I still had the agro of getting the wires into the back door.
Having done it all now though, I love it, it makes reversing up to things a dream, and the night vision means I can see stuff behind me in the monitor, that I can't see in the door mirrors. Not only that, but the camera's have microphones, so when doing things, like checking the lights on the caravan, Suz stands at the back, and only has to say what is on, rather than having to shout it out for the whole neighbourhood to hear.
The Camera
The monitor