Check your grounds. the one on the battery, as well as the ones bolted to the chassis in front of it. if they are corroded and have a bad connection, you will see a loss in voltage, as well as damage to the battery from bad charging.
test your alternator and battery at an autoparts store. odds are, the battery is lost. if your lucky, the alternator is just fine. (assuming the grounds were bad, and the bad battery didnt hurt the alternator)
make sure whatever battery goes back in there is fully charged. you dont want to charge a low battery with an alternator. its bad for both.
2002 GT 5sp. 16x,xxx miles
Steeda Exhaust, C&L plenum, BBK Throttle Body, Steeda Underdrive, Steeda short throw, unknown stage clutch. Rear main leak. Compressor seal leak. Steeda lower control arms. good tires.