2000 Pontiac Firebird Brake Lights Don't work

Using the alternator to charge battery isn't a good idea. Take the battery off and get it charged. Also check the charging system, engine running. Unless the battery has a full charge, your just spinning your wheels.

There is a stop hazzard fuse in the fuse box, not only check the fuse, use a voltage tester at the fuse terminal, should be hot all the time. Also, check all battery connections coming and going on both ends, clean and tight.

Does the highmount brake lite work? If not, check voltage to and from the brake switch. The orange wire is voltage to the stop lamp switch. When the switch is activated, the lite blue wire goes hot.