Because the button is separate from the controller this would be a good time to activate the controller instead of by-passing it. Because the controller is in the steering wheel though you have a problem with getting any extra connections to it unless there are some unused tracks in the clock spring.

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