I have a 1990 300SEL with the gasoline engine. My problem is related to heating. While idling, the heater will blow warm air. As soon as the car starts to move, the air gets progressivly colder until it matches the outside temp. I assume it is drawing cold air in but I do not know from where or how to fix it. Does anyone know how to fix this?

http://hvac.mbzarticles.org/plumbing/monovalve/