VW is really getting on the ball with these techy and wired up cars. If you recall the
clown car VW iFox, it had a nice integrated plug-in for an iPod. This adapter is available for a variety of VW's and Audi's, including used models for 1996 to current. The adapter goes from the stereo head unit to an in-car PC through USB. This allows for full control of the PC's multimedia through the head unit navigation buttons. WinAmp appears to be the only audio playback software directly mentioned at the moment. The concept behind this is to further integrate the in-car PC and ensure better sound quality than an FM transmitter of cassette adapter.
Audi, VW: In-dash car stereo to PC adapter [The German Car Blog]