That looks really cool! I was going to build a similar hard wired device using an arduino and put in the ashtray like that! I even found a small orange display (not easy) but it isn't readable in daylight!

RE: the the DME temp light thing. I was going to check that by changing the temp value while monitoring DME to see when gauge light comes on, just didn't have time yet.

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