I would have guessed the motor first (brushes) then the regulator, but I will not argue with MB DOC... I think the test is to look for voltage at the motor and if there is some then the motor is bad. If there is none then the regulator is bad. Another test would be to apply 12V directly to the motor and see if it spins.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra