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Does it have any smell to it at all? It will either be the brushes or the regulator.
First thing is to test it at the alternator + terminal to determine the output, which im sure youve done. If the output is a constant voltage and is as low as 10v, id say brushes. If its up and down all over the place, yet still not sufficient to charge a 12v battery, id say regulator, and in this case it may have a burning smell, and may even output a constant low voltage.