If it's not too late, Circuit City stocks the adapter in question; strangely
enough it was prominently displayed and I didn't even have to ask the
dead-eyed husks working behind the counter for it ("Vana-what?").
Googling for "vw antenna adapter" yields some promising results (see
http://www.audiowarehouse.peachhost.com/ct_CG13932141.htm).

Best,
d.
1990 GL -- "Mudskipper"