My 300D had a similar problem years ago. I found most of the cold air was coming out of the defrost instead of the center vents. The problem was the vacuum actuator (a large valve and pink in color) that opens and closes the butterfly that directs the air flow to the center vents. A small piece of the valve actuator's plastic broke on the piston that moves the rod connected to the butterfly. Your valve actuator my also be broken because the plastic fits tightly around the rivot that connects the piston rod to the activating arm (later I had to replace another vacuum actuator and the rivot connecting the piston to the rod was so tight it barely moved, I removed the rivot and connected another rivot that allowed free movement of the rod). Or, there may be a vacuum leak to this valve actuator in a vacuum line or the vacuum line fell off. Or, the diaphragm in the valve actuator is cracked, split, etc. Or, there may be some other problem causing the butterfly to not work properly - like the switch over valve that receives the elec. signal to activate the vacuum actuator and changes the vacuum to the element so it moves the butterfly. Or, the plastic check valve has cracked that is part of the control system to the defroster and center vents actuators.

Also complicating things is the fact there are two subsystems that can be causing the problem because there are separate valve actuators/switch over valves - one for the defrost butterfly and one for the center vents butterfly.

More likely it is the center vents subsystem where the problem is. The vacuum actuator for mine was located above the accelerator pedal, behind the lower dash panels, and screwed to the heater-evaporater assembly. Your actuator looks to be in about the same place but higher up and more in the middle of the heater-evaporater assembly - meaning it is harder to reach than mine. I was able to fix mine with a bit of epoxy and this fix has lasted over 12 years - knock on wood. Perhaps another member with experience on your model year can tell you exactly where the center vent vacuum actuator is located.

I will say that these are pretty complicated climate control systems and fixing them can be a trying experience. In my opinion, M-B made the climate control system far more complicated than they needed to. In your case, the climate control system is not as complicated as mine that has the servo controlled climate control - whatever solace that provides. A shop manual is a needed tool to work on these systems - a good one can be found at your library in the Mitchell service manuals (at least my library has the Mitchell manuals). You might get lucky and the problem is simple, if not, then a shop manual is a must have.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.