+1 I think I would be looking for a short (or maybe even a poor ground) in the wiring starting with the harness going to the trailer. I am assuming you don't have a trailer connected ?? Are you really sure that you have all the wires matched up correctly?

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