Check the grounds... There is a write up about adding a new ground on to the engine to fix several electrical gremlins... http://www.fiero.nl/forum/Forum2/HTML/043240.html
Edit: Check obvious things like alternator output. Use a multi meter, the gauge isn't all that accurate. it should be about 14v with the car running.
I also agree tshark, the wiring is old, check the ohm readings, dim lights anywhere means there is high resistance somewhere in the circuit... Ohm out the wire from the bulb back to the various other locations... if you find high resistance, look for corrosion on terminals first before going crazy looking through the harness; simple corrosion on terminals is usually what causes high resistance.
Red 1988 GT under restoration!
( ͡° ͜ʖ ͡°)
[This message has been edited by Shho13 (edited 12-27-2014).]