Radio Unlock Code for 1990 Golf

Hello - I just acquired a 1990 Golf from a very sweet 80 yr old lady who never used the radio (Blaupunkt) and didn't know how to operate it. When I turn it on it it asks for a code. I figured out that it is a 4 digit number, each number 0-9 and I believe that 5 is the enter key. Does anyone know where or how I can find out what the code is? I can't find it in the manual.
Thanks:)

It was on a separate card in with the owners manual. I've seen a few older cars that still had it on a sticker on the inside of the spare tire well. Other than that you'll need the radio serial & the cars VIN (+$30) to get it from the dealer. You push the mode and seek or mode & scan at the same time for 4 seconds to get to the code entry screen & to enter the code. The radio presets 1 to 4 set each digit in the code.