Is the present radio in a 1 DIN sized opening? (2" x 7") If so, then any 1 DIN size stereo unit will fit in place of the old one. If your dash doesn't have a 2" x 7" rectangular sized opening, but only small holes for the knobs and cassette player, probably the dimensions of the radio behind the dash is still about a 1 DIN size. You could remove the radio and cut out an opening for a 1 DIN mount and slide in a new unit.
Look at all options available in a new stereo, I like the fact that my stereo allows input from my iPhone, charges it, and also has a tiny wireless remote I have in reach on my steering wheel spoke since the radio is a bit of a reach.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI