Like JDUB said - before changing alternator try regulator- if it is a Bosch - on the back of the alternator is a black disk about 1 1/4" round with a flange and two 1/4" bolt head screws holding it in. Take out the two screws and tilt the assembly towards the center of the alternator to remove it. If the brushes are very short - definately the problem. On new unit they are about 3/4" long. Changing this regulator assembly is the cheapest and easiest thing to try first. Remember to disconnect the battery before removing the regulator.