The X5 has a seperate grounding wire since the X5 TV mounting brakcet is plastic. I do not recall in the X3 if the bracket is plastic, but if it is plastic, you will need to ground the TV video module to chassis ground with a BMW grounding cable. The cable attaches to the screw on the top of the module on one side and the chassis ground on the other.

Also good point about the rear backup camera system. You can NOT have the power for the camera shared with the power for the backup relay. That is a known cause of noise. The backup camera needs it own seperate power