Hi Paul, :welcome: to X5world
The way this actually works is via a message that is streamed on the i-bus that tells the electronics in the vehicle when you are exceeding 5 MPH. At this speed the TV signal to the vehicle is disabled and all you are left with is the audio.
What you need is something like the TVonNav bypass module. The module goes in-line with the cable that goes to the video module. The other thing this will allow you to do is continue to listen to another audio source (such as the radio, CD changer, etc.) rather then the AV source (which in your case will be nothing since you will have a camera as the source).
The TVonNav module can be ordered here -
http://tvonnav.com/bmwmodule.html
You may also want to consider buying the AV Connection Cable that is also available on that site, since this will give you the connection point into the video module for the camera.
Jeff