Several radio models were used in the Fiero. Listing all the variations and the cars which used corresponding radios is pointless since OEM and aftermarket replacements are readily available. (For a used radio, check late '80s GM cars in any salvage yard.) One of the options available was "Premium Sound" with a sub woofer. As far as we can determine no other vehicle used the same sub woofer system so replacements will have to come from a donor Fiero.
Newer GM radios which will fit the Fiero and maintain the factory look can be found in salvage yards and via the Internet. The radio/CD unit from early '90s Pontiac Pontiac Grand Ams is a popular choice. Wiring adapters or modification of the connectors is required. If radios are salvaged from other cars be sure any security features can be bypassed. The Fiero radio size is "1.5 DIN". Many aftermarket radios are made to this size and will fit with few modifications.