I was looking around for exactly that yesterday. I actually got mine from a VW dealer, the technical guy was very helpful and we pulled up the wiring loom of the '98 Jetta and ordered about two 'likely candidates' that were in the general area of the headunit. I stupidly forgot to write down the part number and was hoping I still had the receipt somewhere.

I did find the bag with the wires that go into the connector, so I have that part number. But it isn't as useful as having the connector body. It is a standard ISO-A connector, it's distictive feature compared to the ISO-B connector which carries the speaker wires is that this one has a tab at the bottom, while the speaker connector has the tab in the middle.