Clay - If the more traditional fixes don't fix it, take what Jim explained to heart. My '81 380SL did that for a while and I finally traced it to a crack in the foil trace on the back of the instrument cluster P.C. board
Barry The charge light is hooked between the field and the battery. If the alternator is not generating current the light supplies field current. If there is a bad connection to the battery, current will flow backwards through the light, also lighting it. In other words, it could be any number of things. A voltmeter and an ammeter are really required in order to be able to pinpoint what is truly going on. I'd check the brushes first, the belt tightness, and the state of the connections between the alternator and the battery.
