S600 Heater problem
 

I have a 92 S600 which has a heater problem. There does not seem to be any hot air coming from the under-dash vents. If I rotate the temp wheels to max hot, the heater fan speads up and the air gets hotter, but no air from lower vents. Now if I turn the temp wheels to max cold, the lower vents open, and hot air is directed to the footwells, intitally hot, till the air con takes over and cools it.
My question. What is stoping the footwell flaps from opening, and why do they open when cold is selected.

btw the engine temp is as it should be.

Many thanks for any helpfull advise.

Bob