My 1987 560 SEL has no hot air on driver side floor. Passenger side is ok. I pulled the control panel and checked the floor control actuator and it is working. I pulled the left side duct work off under the steering column and the door was only opened 1/2". When I move the floor actuator it moves the lever for the left side door but the door does not move very much. I assume the door is broken. It looks like a tough job, any advice? Thanks.

Any internal flap repair requires complete disassembly of the heater box. Sorry to hear this, I guess thats why MB offers a repair kit for the flaps.

I dug deeper and sure enough the top hinge pin was broken. I'm guessing the foam wore off the door, exposing the adhesive which then stuck the door to the housing. After a period or time the hinge pin failed.
I'm not a big fan of pulling the entire dash assemblies out of cars. They never seem to go back in as well as the factory installation. My plan is to replace the door without pulling the heater housing. It's worth a try. I'll let you know if I succeed.