BRN/GRN for the washer level is the signal wire which when grounded by the level switch with the BRN/BLK wire keeps the washer level light off. NOT saying that is the wire you are dealing with but that's what I can find on my 3.0L just to give some idea.

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