Nic,

I can second Bob's comments. My center vent servo on my '85 300 td went bad and I was able to fix it without removing the dash. In my case the rubber bladder inside the servo split and caused the vent to act erratically, opening sometimes but not others. My car is in a fairly hot climate so I think the heat got to it. I lived with it for several months until the system couldn't maintain enough vacuum to work the locks. The vacuum system sounds really complicated but it's really not if you have a cheap vacuum pump and tester. You simply isolate one line at a time and test to see if it will hold a vacuum. I bought a cheap plastic Mighty Vac at the local parts store some years ago to help bleed my brakes and while it proved to be worthless for bleeding brakes, it's great for testing MBZ vacuum systems. Go figure. The vacuum servos aren't that cheap so it's worth the investment just to avoid unnecessarily replacing a vent servo. Good luck.