X2 on what platt-deutsch said. I would like to add that you can go from the neg battery post (not cable end) to the alternator case. If you see a voltage above 1/2 volt then start moving the meter back closer to the battery. For instance, check from the battery to the block, then to the body, etc. When the problem doesn't show up on the meter, you just passed the problem. So you know the problem is between the last bad reading and the good reading.
If the ground doesn't show a problem, then do the same thing with the positive. For instance, check from the battery positive to the alternator output. If you see a voltage drop there, start going back toward the battery.

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84 500 SEL (307,xxx miles)