I've got the same thing on my Peugeot 406, mod. 2000.

There's a factory tapedeck controlled via buttons direct on the unit or with a steering wheel command stick and with a multifunction display in the dash (Displays temperature, time, date and radio and tape controls if switched on).

I already tried to figure out how the control-stick works and for details of the tapedeck (It has an ISO-connector which seems to control the controler-stick and the display, too). The only thing I got is something like an electronic-plan how the display and the control-stick are controlled and integrated. As I can see it's something like very specific and nonstandarized communication.
There's something like a central unit ( named 8511) and the display and the stick are somehow connected to it. and there is another thing (called 8500) connected to the same line of the central unit which make the connection to the radio but not directly but over something like a decoder.

This information is very poor but I didn't get anything more and I don't really know what I should do with it. I could guess that this is something equivalent to a serial connection but I don't know. It could be something else, too. And I don't want to blow up the car's electronic just because I'd like to connect the control stick - and if possible the display, too.

I think in the audi and the vw's it's something simmilar. Anyone out there who knows?

TeeMcBee