The LCM modulates brightness by pulse width. If you are using a micro controller then in theory you should be able to detect on/off and brightness and output accordingly. I think there are 2 or 3 different brake/tail lite strategies though so keep that in mind.

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