I talked to my local dealer, he will print me the wiring diagram for the cables that run to the radio so that I might be able to make my own interface. I'll post if I find something.
Up till now I only got the screen mounted where the old radio was and another radio down where the cigarette lighter used to be. I'm not satisfied with both installs so I will do everything again over the next weeks. (See here for pics of what I want (left) and what I already got (right)).
So... I did get the wiring diagram and I did make an interface, but only for the steering wheel remote controls.
I've used a K8055 card and a voltage divider. The K8055 accepts voltages up to 5V and it has 2 analogue inputs. There are jumpers to switch between internal use (i.e. the analogue input is powered with 5V internally) or external use (no power from the card). So I used one of the analogue inputs to get 5V and used the voltage divider to change the voltage that I detect with the other analogue input. Very simple :-) (See here: http://www.alexander-bock.eu/carmac/...en/Bild46.html)
That is the diagram for the steering wheel remote. I hope I could help. The diagram from the workshop has some other pins, too, but I had no use for them (speed, keycode, mute, etc.)