Let me know how this goes. Lately, I noticed my W124 temp gauge reading approximately 20 degrees below actual as well.
I figured I'd pick up the $50 part from Fastlane and be done with it. Perhaps this isn't going to fix the problem as I previously thought?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car