I just finished exploring under the dash.

I found that the cabin air temp sensor air pump is missing, but it still seems to work. I cant find the power wires for the pump.

All the dash vents are supplied from the same air box down stream of the AC evaporator and I think behind the heater core. That explains why the air temp is the same regardless whether coming out of the center or outer vents.

I also found that it has the doors to select out side air or recalculate inside air like my 126 will, but there is not a switch to control it.

I agree that the mono valve is most likely the cause of the constant heat because there is not a fresh air source to supply cool air to lower the temp of the air being blown into the cabin. My 78 240D center vents are a separate fresh air source where as I can have the heat on (the outer vents) and have cool air coming out of the center vents. That is not an option on this vehicle.

Unless my car is an odd ball, having heat come out of the center vents is just a matter of turning on the valve to supply vacuum to the door actuator when the heat is on.

All my vents are closeable, which is nice.