damn, just a few weeks ago there was a great thread about alts. it was started by sponge. here:
At wits end... alternator...
if your light is on all the time without the ignition on. it gotta be the blue wire on the back of the alternator. look at the thread above. some smart folks jump in that one. look at john cramers' and loren's replies closely. i bet you have an easy fix.