My understanding of the W126 and the diagrams. Someone correct me if I am wrong. Cool/Fresh air is all that is available from the center rear vent. Rear heat is ducted to the feet only (there is a duct system that runs under the front seats).
If the control lever at the rear center vent doesn't shut the cool air off, then more than likely a clamp has popped loose from one end and the cable jacket is not anchored. The other end of that cable can be seen by pulling the glove box and looking at the right side of the center plenum with a flashlight. (I just had mine apart a couple of weeks ago for a pod repair.) Who knows you may be able to fix it with minor disassembly!
