I have a procedure I like to follow if I am working by myself and chasing a wiring issue.
When I know there is a short circuit (blows the fuse twice on the same circuit) I use a self-powered test lamp to help troubleshoot. To use this lamp, disconnect the battery. As with an ohmmeter, the circuit has to be dead.
Then start at the far end of the harness, say right hand brake lamp socket, and clip the lead onto one of the lamp socket terminals. Make a good connection to ground with the probe, and jiggle the harness from rear to front until the light goes on (indicating short circuit made) or off (indicating short circuit broken). Continue working the cable from one end or another to isolate the fault. Two common locations are license lamp wire next to trunk latch support, and alongside rear seat.
If I have an open circuit, lamps don't light but fuse does not blow, I use a plain 12v test lamp and check the harness from fuse box clip for the appropriate circuit to good ground, from the switch input to ground, from the switch output to ground, and from each socket in series to ground. Done with the battery connected, good wiring will light the bulb.
Many Firebirds have grounding issues in the rear lamp harnesses. You can make a temporary jumper wire to attach to the cast metal housing, and to a bare metal ground point. For a non-lighting circuit that does not blow fuses, this is sometimes all it takes to change a dull glow or flickering bulb into a proper flasher.
