You'll need a cable that fits into the MP3 player inputs in your VW (most likely a minijack) and then connects to the iPod.
Connecting the iPod has two options: from the headphone port, or the dock connection. The dock connection will give you better audio quality.
You'll also want to power/charge the iPod via the car, so getting a connection to do both (power and audio) is important.
See this post for a start:
Buegie, "connect to my stereo?" #4, 09:19pm Oct 7, 2005 CDT
Belkin also makes an auto DC adapter that powers/charges and has a line-out feature (about $40), Then all you would need is a male-male minijack cable.