It is likely a problem with the regulator rather than the alternator; however the the alternator may be weak at the same time. The regulator is on the back of the alternator. Two bolts and it is out. The new regulator is sort of tricky to get back in since the brushes on it are spring loaded and have to be sort of compressed to get it in. In total the job took me about 20 minutes to do.
The regulator is a $30 fix, as oppose to $300 fix for the alternator. You should check the output from the alternator after a new a regulotor is installed to see if it is still good. I believe it should put out about 13 V.