Ford Repair: 96 Mustang GT Charging system, mustang gt, charging system

Question
My mustang has a problem in the charging system, when I turn the AC on the amp gauge drops and keeps dropping as long as I am using it. It seems to be charging fine except when I am using the AC or heater and only when the blower motor comes on does it drain the battery. I checked it with my Fluke and the alternator is charging a around 13.5 volts DC. I hand the battery tested at the parts house and they said it was bad. I replaced the battery and it did not help, the car is still acting the same. The AC seems to be working fine but it just drains the battery. The only thing wrong with the ac is the fan resistor is out and it only works in high, but when I put it on in low the compressor kicks on and the battery does not drop, just when the blower kicks on. I've replaced both battery terminals and stripped back to good wire and that seemed to help a little but not fix the problem. I also installed a few extra body grounds just to be sure that was not the problem. I am at a loss, the only other thing I can think of is that the blower motor is stating to short out and is dragging the battery down.

Answer
Based on what you've said, I would be looking at the alternator.  13.5 volts is on the low end of "acceptable" and running the A/C puts a huge strain on the charging system.  It is very likely that your alternator just can't keep up with the demand anymore.  Normal operating voltage with the engine running is 13.5 to 14.2 volts, see what it is at idle with the A/C on.  If the blower or any other part shorts out and draws too much power, it will blow a fuse.  Good luck,
Jason