I have a 1992 Ford Mustang GT. The car won't hold a charge or charge the battery good enough to start the car after I turn it off. The battery is now and the alternator is new.
Recheck your work for the alternator and battery installation paying particular attention to possible areas of corrosion and polarity of the wires. If they all check out, I would suspect that something is draining current when the car is off causing the battery to loose its charge. Double check all incidental things like courtesy lights, amplifiers, etc. to make certain they are OFF when the car is at rest.
Search Ford Mustang Questions
Ford Mustang Experts