Adding a subwoofer is easy.
First you could locate the Fiero "Performance Sound" option on another Fiero and install it in your car. This is the factory subwoofer system. It mounts under the passenger side dash, has a small amp that mounts on the center console, and has a level control that mounts in the overhead lights. They appear on Ebay occassionally.
Second, some make a small box with an 8" sub and mount it under the dash on the passenger side. (This is what I did)
Third, there is a company that makes a box that takes the place of the center console glove box that fits an 8" sub into it.
Finally there is a website called Fierosound (fierosound.com/stereo.html) where a guy describes mounting 2 10" or 12" subs behind the seats by cutting into a hidden void located there.