In reply to sweenyt:
I'm wracking my memory but on a 55 plate I don't think it would have been standard, VW were only doing iPod adapters and CD changers in that age of car. We only got the fancy stuff like SD cards about 4 years ago.
Assuming it is the standard radio unit (RCD300 I think, check in the manual and you should have a book that tells you which unit it is) all you should need to do to access it is press the CD button twice, this then activates the auxiliary input which in that age of Passat would have probably been a 6CD changer mounted where your fancy thingamajig is.
Mark
VW salesman for almost a decade