On the 1985 123 cars, I think the damper controls with two hoses are meant to open when they receive a signal from two different controllers. For example, the side vents are open whenever the system is calling for defrost and whenever the system is calling for air conditioning. It has been a while since I saw a diagram on this, but that is my recollection.
I love both of my Mercedes, but they have the worst ventilation system of any car I have ever ridden in with the possible exception of an early 1960s VW Beetle I used to own. The Mercedes systems are unreliable, complicated, and of general poor design.