Hello Presse47,

What Zacharias said is more than likely the problem. I have a 1981 W126 but it is more than likely the same. On my car I had exactly the same problem and the cause was the disintegrating foam hose. On the W126 it is about 15cm (6") long and connects the fitting on the base of the temperature sensor housing to the Neoprene hose that connects to the blower motor. When the blower runs it pulls a small amount of cabin air through the small grille on the dash and past the temperature sensor. It then signals the control module to manage the Mono Valve.

On my car I could just get at the hose by removing the instrument panel. You will just be able to see the base of the temperature sensor housing. As soon as you touch the hose it will crumble in your hands. As Zacharias pointed out a piece of water pipe insulation 15mm (5/8") will fit quite nicely. As mentioned, 15cm (6") should do the trick. Don't forget to glue or tape the joint if it is the split type of insulation.

Hope it helps,