So yes at this point it looks like it's a poor battery positive connection, BST, (most likely), or bulkhead splice. Awesome to have a diagram to work from! If you have test light it would work just as well and might be easier to prod with.

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