I also suspect that these rear cameras are aftermarket installed.
When I put the Eonon GA6166 unit and rear camera into my X5 I experienced shaky and intermittent camera images.
I finally learned what others had learned in that picking the voltage off the reverse lights directly to trigger the camera picks up some really weird voltage monitoring stuff going on by the BMW brain.
The solution others have suggested and the one I also employed is to use a relay to isolate the camera "signal" from the actual reverse light voltage feed wire which has all that BMW brain stuff going on. Here is what I did to get a better rear camera experience.
https://xoutpost.com/1100757-post199.html
Mike