The AV-IN shield (ground) inputs are isolated from the car chassis.

When you are using a DVD plaer powered by the vehicle, the DVD player is connected to the chassis of the car and everything works fine

If using a separate iPod or video player which is a portable device, then you will need a ground connection.

BMW design it this way to prevent ground loops which cause video interference.

If you only use portable video devices that will not have a permenent ground conenection, then you may want to connect the AV-IN shield to ground inside the TV module white plug.