Well, luckily with a Fiero being the size of a Fiero. Not too many places to hide an aftermarket amp. Look behind and under the seats.
Factory DelcoGM radios in a Fiero only came with an internal amp. Unless it has the factory Performance Sound. If you have it, there will be slider switch on the overhead console that says "performance sound." The amp for it is behind the carpet on the right side of the center console. Pull the carpet back and it wil be screwed to the plastic panel that's between the carpet and console. It's a small box that's approx. 5"x5"x2" Hope this helps.