[Gearhead999]

The battery may be bad, age really doesn't mean much. Plus, if the mustang has a generator vs an alternator it isn't getting much of a charge at idle. Generators don't put out much at idle.

Get a "Battery Tender". It is like a small charger, but won't overcharge a battery if left on the battery for extended time. Even a "trickle charger", which is usually a very low amp charger will overcharge the battery when left on. The "Battery Tender" maintains the battery at a voltage level just below where the water starts to boil off.
They're cheap to $50-60.

Quick question....what about the gas?? It is getting old and running the car doesn't change the age of the gas.