Grounds are just as important as the positive connections. The best way to check is by looking for a voltage drop (that should have done by the mechanic who checked). There is a good post a few threads down that shows how to do that.

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