I'm trying to figure out the seat-back vacuum on my 85 300CD. The seat-backs are supposed to lock in place, but I can't figure out how they're supposed to work. The manual diagram (80-902) shows a "door contact vacuum swich." I'm wondering if this is the same thing as the interior light switch in the front of the door. It also shows a "vacuum switch" for the backrest and a seat vacuum "element" with an actuating rod.

Does anyone have experience with this system? All I know is that mine is not working (I don't think--very confusing).

Thanks,