1992 Pontiac Sunbird

Electrical problem
1992 Pontiac Sunbird 6 cyl 69000 miles

I have a 92 Sunbird that will not charge. The alternator has been replaced and I have tested it twice on a machine at my local parts store. The old alternator check good as well. The car will not charge the battery while I'm driving and only intermittently when idling. You can see and hear the RPM change when the chargin system " kicks" in and the amp gauge goes up into the safe zone. The lights also brighten as a result of this event. I have replaced the positive battery terminal and I've tried two different batteries in the car as well. I have checked the plug coming into the regulator and looked at the wires as far as I can trace them (12" ) and I can not find anything wrong.
Any other ideas? Any help is appreciated
October 11, 2010.

Here's what you need to check while the car is running and not charging. Check at the alternator the red battery terminal it should have battery voltage. Then check the smaller red wire terminal S it should have battery voltage. Then last the brown and white wire terminal F should have battery voltage.

Oct 11, 2010.
Is there a charge lite on the dash, does it work? Use a digital multimeter across the battery posts, everything turned off. The generic spec for a full charge is 12.6 volts. Then, take reading at alternator bat terminal, everything turned off, the readings should be the same, if way different, replace the wiring circuit.

Oct 11, 2010.