So you have one of these cables?
http://cgi.ebay.com/VW-Audi-Skoda-COM-OBD2-II-KKL-DIAGNOSTIC-CABLE-VAG-COM_W0QQi
temZ160074446119QQihZ006QQcategoryZ43989QQrdZ1QQcmdZViewItem

Read here:
http://www.ross-tech.com/vag-com/faq_1.html#1.1
If your car has the old "2x2" Data Link Connection, check if there is a wire
going to each pin, as shown in the picture. If there are four wires, VAG-COM
should work.  If there are only three, it won't.
According to the ross-tech.com site it may work for your '93 Audi 90! :-)
http://www.ross-tech.com/vag-com/cars/applications/90cabrio.html

Try out the limited version to see if it works.  Just download the correct
version that is for your adapter and if it works then you can buy it to
unlock more features of it.  ;-)
http://www.ross-tech.com/vag-com/download/

later,
dave
(One out of many daves)
http://vwdoc1.tripod.com/