From one of my previous posts: "Had the same problem with a couple of my vents (1986 2.3-16V). I ended up purchasing ALL new vents (qty: 4). The price was pretty reasonable at my local MBZ dealer -- about $9.00 each. Other than that, I was told you could remove the vents and glue a small strip of felt to either of the sides to create more friction. To remove the vents, get a couple of needle-nose pliers and a rag (to prevent gouging the vent). Grip a vent blade near each end of the vent and pull straight out. To install, just line up the slots with the pegs and push in."
Hope this helps.