Okay, here's what my little VW Radio Card says for my '91, (in so many
words):

Before entering your code, (the word SAFE should be on the display), press
and hold the am-fm button and while doing so, press and hold the scan button;
after about a second the number 1000 will appear, at which time you can start
to enter your code.  It warns, however, that by holding the two buttons down
too long or by pressing them a second time, you will change the code to 1000
permanently.

Well, it stands to reason if you hold them down, your new code will in fact
be 1000; then all it says to do is press the am-fm and scan buttons again,
the word SAFE will reappear, you let go of the buttons, and you should be in
business.  Hope this helps.

Best,

Sam