I had the exact same problem.

I have an Archos AV380 and I couldn't make it work even with a correct 3 lead AV minijack cable into the "AV In" input in the rear console.

Video was dropping in and out, and the TV-module then mutes the audio.

I had a carstereo installer have a look at it,
and he found out that the Archos and the BMW TV-module needs to have common ground in order to sync up the video signal.

If you use a car charger for the Archos at the same time, there shouldn't be a problem, but if you are running the Archos on batteries, there's no common ground.

So he made a special cable that is grounded from the 12V output (sigarette lighter) in the rear console, and now it works with the Archos on batteries aswell; video and audio are OK.

But you only get mono audio, because of the mono tuner in the BMW TV-module...
(The cable was made so that left and right channel was merged.)