OK, I actually got into the harness(and put a new battery in my multimeter) and did some more serious testing and I was wrong about the wiring diagram being wrong! Sorry guys, but might be good news. The light side of the sending unit appears to be indeed a NTC resistor although with a completely different resistance value than the DME side. So might be entirely possible to tweak it as Andrewwynn suggested. Will keep you guys posted.

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