depending on the year model you have you will either have a brake cable assmbly under the rear muffler (84-87 Fieros) or inside the K-Frame railing in back of the Cat. (88 Fiero).
In either case, the adjustment procedure is the same; you have an adjustment wheel that is about 6 inches long and shaped like a metal frame tube. At one end is a nut (15mm if memory serves). You want to hold the wheel with a set of pliers and tighten or loosen the nut as necessary to increase or reduce tention on the cables. There are some pictures of the cable of the 84-87 Fiero in this thread:
https://www.fiero.nl/forum/Forum2/HTML/088599.html