By the way, the adapters to fool the CD-changer interface into thinking there is something there are available like somebody posted. I think they go for around $80. And if you were so inclined, you could build one yourself with a microcontroller for a whole lot cheaper than that, which is what i did (I lost the firmware that i programmed it with, but i think somebody else has some available on the net).

But I think you're probably better off just going with an aftermarket deck unit and not even bothering with the VW single-DIN.