I have an 84 300D, when I got it the drivers door panel was off and missing. The solid yellow vacuum line at the bottom of the door is missing a chunk of about 3 inches of line, the other 2 lines go up and connect to a white parts that is connected by a small linkage to the rest of the door lock assembly and to the push button. This small white piece looks nothing like the central locking system locks I have seen before or on E-bay. The 3 lines hook to the bottom of it and the piston and linkage part come out the side of it. Will try and upload a quickly scanned pic of this in hopes some of you experts can identify this...The door doesn't appear to have been replaced from an earlier vehicle, the mfg date is 10/83 on the door pillar....

That is the correct part, it's called the master vacuum switch:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41V018Y6KZ&year=1984&make=MB&model=300-DT-001&category=All&part=Master+Vacuum+Switch&appChassis=_any

The solid yellow line connects to the center connector, and the yellow/red (lock) lines connect to one end. The yellow/green (unlock) lines connect to the other end. This switch causes the other door locks to "follow" the position of the drivers door lock.

This diagram may help:

http://home.comcast.net/~phantoms/vacuum/interlock_system_layout_1_0203.jpg

Thanks for the info Craig. Upon a more careful exam of the part I found out why it was malfunctioning. The shaft was bent ans thus it would not go in all the way. I put it back on, cranked her up and with my spare key locked the door and all the others followed suit. But now I have a vacuum leak somewhere, as I have found if I have both yellow lines hooked to the little yellow check valve under the hood, next to the brake booster, that the central lock system will work, but the engine won't shut off. When I plug the yellow line that goes to the drivers door from the check valve, the engine shut off works great, but no central locking....Got to get out my vac and see if I can trace down the lines.....

Sounds like you're on the right track. BTW, it's easier to test the portions of the system at the branch connections in the vacuum lines under the front floor matts than taking all the doors apart. Start at the connections on the drivers side and work your way out from there. Have fun.