Take both the battery and the alternator out of the car, then to a parts store for testing. Don't bother with Sears or anything. They charge to test, most parts stores don't. Just make sure you find the most competent counterperson to help you. My bet is that assuming ALL of your wiring is correct (double check it all with a continuity meter), one of the two will be bad. Good luck.
------------------
Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch