This just started. with every thing off except the engine running at idle the voltage at the battery is at 13.6volts I turn on headlights rear defroster stereo heater and brake lights and it drops to 12.5 volts and continues to drop. If I turn every thing back off it goes back up to 13.6volts I have now replaced the alternator and battery. With everything off except the car running at idle off the alternator is at 13.9. Any ideas on what I can check?

 

12.5 is not bad on a 25 year old charging system.The output of that alternator is not exactly Hi capacity!..(70 amps)
You can always look up an alternative Higher Output alternator such ad the FD alternator.(100 amps).Members have been installing Ford Taurus alternators too at 130 amps.(search that.)

 

That sounds good I racked my brain on this all day yesterday.