Thank you very much for your reply and for the helpful link. Unfortunately my car wasn't prepared for either BMW assist nor phone. I installed the wires as a part of the retrofit. And it wasn't the same retrofit-set as yours.

The 'funny' thing is that everything is working with the cassette player. So in some way I'm quite sure that the wiring is okay, except the audio doesn't come through. I suspect that some signal doesn't come through to the business cd, telling it to let in/out the audio. How will I know if TEL-ON is coming through to the radio - to the x13321 connector?