I have a 1984 300CD that only has heat only when it idles, although not hot heat (lukewarm). As you drive the car you have no heat at all. Heater core is not stopped up, heater valve is working properly, electric pump is working, but buttons on console make no difference. Air only comes out at side vents and window defroster vents. Air cond. works fine but it too only comes out at side vents and defroster vents. My wife's feet are freezing and it's supposed to be 14 degrees in the morning. Any direction on this problem is greatly appreciated.

Be sure that you have a new mono-heat valve insert.

Also, the vacuum getting to the system is critical.

I have seen small solder cracks in the pushbutton control units. You may want to have the system checked by a pro.

I once had similar symptoms...it turned out to be a dirty fuse.