I made a modification to my '85 300D that I think is probably one of the best free mods you can make on the 300D with the automatic climate control system. I totally removed the damper door and vacuum actuator for the center vents in the dash. The dash does have to be off to make this mod but in my opinion it should have been done like this from the factory. If I want the center dash vents closed I will do it manually using the dash vents. You do have to make sure they work. Works great and is free.

I took the vacuum hose that went to the center vent vacuum pod and put it straight to the 1/2 section of the defrost vacuum pod. Not sure what it's purpose is but the system works great.

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Jim