I have a 86 300e. The opening and closing of the vents are vacuum controlled. Hopefully you just have a vacuum leak, check the vacuum connections on engine side close to the firewall, drivers side. Otherwise, the vacuum actuators have a rubber diaphragm that, with time, go bad. The ones at the foot wells can be replaced by removing the console (I had two in mine). For the one that controls the center vent, the dash has to come out. MB sells the whole unit but, I found a source that sells the rubber piece only, A LOT cheaper. I used the printed manuals to replace them. However, I had to figure out many things on my own.
Good luck!

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J.H.
'86 300E

[This message has been edited by J.HIDALGO (edited 10-12-2000).]

[This message has been edited by J.HIDALGO (edited 10-12-2000).]