What's funny is that I sat in the Advance Auto parking lot and changed out 3 alternators before giving up. Took it to a local Mr. Alternator and they couldn't figure it out. Finaly once of the gurues came in and said check the fuse.
A freaking fuse was keeping the alternator from charging the battery. Change the fuse out. That is probably all it is. I keep spare fuses like crazy now.
Did you check the battery to make sure it doesnt have a bad cell. I had a chevy citation that use to eat alternators every couple months. I even bought a new battery right after i changed the Alt the first time. After the third alt i said screw it and took the battery back to NAPA and found out that my new battery had a bad cell. Problem solved.