I just replaced the foam tubing in my '82 300D yesterday. If you take out the glove box liner (by removing the plastic "plugs"), the foam tube will be visible just to the top left of the glovebox hole. It looks like a piece of foam around the tubing, but is in reality the connection between the two tubes - the one on the left goes to the "thru the dash" ambient sensor, and the tube on the right goes down to the heater motor suction side. The foam will deteriorate and split, causing little or no air flow through the ambient sensor, which makes the heat control either nonexistant or very sluggish. When I touched the foam on my car, it broke into several pieces. It only takes a few minutes to replace the foam - good luck!