Ah - well here we come up against another problem. I may have bought the car in the UK, and it has a plethora of pins in the connector, but some are labelled in something which isn't English, like "S Kont","K-Bus", and the puzzling "Ga".

The stereo has a CD select button on it, which implies it can select from a source other than rf, and I guess the most likely candidates for input are the "AF-R" and "AF-L", so I'll probably experiment with them.