I have a problem with the heater in my W124. I have the manual-style system with the two temperature wheels, two knobs (fan and floor/defrost etc.) with the buttons in the middle.

The problem is that when it's cold outside and the temperature is set to something else than full heat, I never get heat. If I turn the temperature dials to max heat, wait a few minutes and then back down, it's ok. Any BTDT?