I just replaced my OEM stereo with Navigation to an aftermarket Alpine that required a male Motorola antenna plug. The antenna adapter I used was Metra part number 40-VW12 that you can get at Crutchfield for $4.99, their part number is 12040VW12.

Antenna Adapter Connect your factory antenna cable to any aftermarket stereo Works with a variety of 1986-up European and American vehicles at Crutchfield.com

For the main wiring adapter, I used the SCOSCHE VW01B 1986-Up Volkswagen Wire HARNESS. I bought that here...

$4.95 SCOSCHE VW01B 1986-Up Volkswagen Wire HARNESS

You will also need 2 MERCEDES AUDI VW radio removal key tools. I bought mine on eBay, but I believe Wal-Mart usually carries these...

4 VOLKSWAGEN MERCEDES AUDI VW RADIO REMOVAL TOOL RRK132: eBay Motors (item 360259571563 end time Jun-04-10 01:18:37 PDT)

Anyway, the above parts are what worked for me. Your mileage may vary.