But Mad North-Northwest
Join Date: Sep 2003
Location: SF Bay Area, CA
Sounds like it's not the stock radio then. Probably you need a tool like the one at the link below, but you might get more info if you search on the specific manufacturer. The '91 originally came with an Alfa-branded Clarion unit.
Universal OEM DIN Radio Removal Tools at Crutchfield.com
1963 Giulia Spider (1750 engine)
Former: 1987 Milano Gold