Have you swapped the bulbs yet just to make sure its not the bulb? If not it sounds like it is a wiring issue somewhere or you might have a weak alternator. Or it could be a bad ground. Remember, your car is a classic in many states already, if the grounds have never been maintained they corrode and ALWAYS cause problems. Foxes also are notorious for the alternators going out. Even if your alternator isn't bad its a good idea to upgrade to a 3G.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.