In answering another Bolters question about a
"defrost cable" I think I stumbled across the answer to your question.
Below is an enlarged section (rotated on it's side to better show the normal position of the heater/defroster control) of one of the pics I posted in that thread (from a 1950 Accessory Manual) that looks like it displays the cable/control and internal "flap" positions.
Looks like when the control knob is pushed in with the control arm in about the 5 o'clock position, the damper flap is "closed" (pushed up to seal off the duct openings). Pulling the control knob/cable out should move the control arm to about the 7 o'clock position and pull the damper flap down, opening the entry port for the defroster feed tubes. If I am reading the diagram correctly???
If you have very restricted air flow to the defrosters, it's possible that damper flap is stuck in the closed position and the control rod shaft is just spinning in the damper flap hole.
Unfortunately sounds like you are going to have to do some disassembly to further diagnose the problem and find a solution.
Come back and let us know what you find out and a fix if you are able.
Dan
