This goes back a couple of weeks with the GT. We really like driving it in the fall so had it in the garage going over things. Replaced the washer pump and re-aligned the washer nozzle. That was a new novelty. Reset the cable to the heater valve and picked up some warmer air. Replaced a seized wheel cylinder which improved the brakes greatly along with the parking brake. Had it in the garage for a few days but when I went to start it, only clicked and wouldn't turn over. Charged the battery, started up and checked the voltage on the Delco alternator. Everything looked good. So we drive it to the club meeting last night. Coming home with the headlights on, radio going, heater and threw in the rear defroster to see how many lines were working on it. Going good for a few miles till the turn signals were slower, heater stopped working, dash lights went out and finally just ran out of power and quit. Luckily the wife (who was driving) was able to get off onto the shoulder of the road, as we were halfway up a very busy 3 lane hill. Good old Auto Club gets a flat bed there within 45 minutes and get it towed home and back into the garage. Popped the hood and start looking for a problem and when I reach down the fan belt is loose enough that I am able to turn the pulley on the alternator by hand !! Not a good thing. I am presuming with the heavy load we had on the way home, the alternator stopped charging and we were running the battery down. I never thought to check the belt when it wouldn't start in the shop as it was showing over 14 volts at the alternator, WITH NO LOAD ON IT... All in all, an interesting night and can't blame the LBC for not getting us home on this one. Putting it down to bad maintenance.