Your best bet is still to take it back to the shop that did the alternator. They will probably have it figured out in a few minutes and charge you for an hour or two but you will be better off. Then when you have time you can study up on electrical and using the multimeter. There should be some electronics training kits available that can be really helpful with hands on practice instead of watching you-tube videos.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)