Good Alternator Won't Charge - Please help

[CSJ3 notchback]

I have a 1990 Notchback 5.0, I just put a new Interstate Battery in it and have been running a PA Performance 3g alternator in it for about 2 years with no problems til now. I left it out in the cold for a weekend and when I started it up the volts were low. The battery has been checked at 2 different places and is good, the alternator checks out good and all the large battery/ground/alternator wires have been replaced in the last 2 years also. Connections are clean, battery light stays on while running and I checked to see if it would go out when I unplugged the wire from the alternator and it did so I assume that circuit is fine. Something Is there another "exciter" wire I should be checking??? Any help would be greatly appreciated, I need to get it fixed asap.