Well turns out the red gauge light gets turned on by a switch in the dual coolant temperature sender/switch. The NTC resistance portion feeds into the DME which works the gauge over whatever bus that is on and the NC switch portion goes directly to the light. So no coding that! ;( but would works out nicely for someone who wants to splice in an Arduino or voltage comparator to work the light.

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