Does full air always come out of the defroster duct, no matter what the climate control is set too? Then yes, I would bet the diaphragm is bad. It defaults to wide open. Luckily it is replaceable without removing the whole dash (though the factory manual says to do so). If you take out the instrument cluster, you can see it on the right side, a colorful plastic pod. At this point, you can hook up a hand vacuum pump to check it's operation. There are actually two ports on the pod as it actually can move the vent fully closed or just partially. Both should hold vacuum and move the lever.

If it turns out to be bad, as far as I know, you need to buy the whole pod, which will run around $50. It is available from the Mercedes dealer or other aftermarket sources, such as this site. To remove it, you will need to grind off the rivet between the pod and lever, such as with a Dremel, and find a bolt or rivet for installation. You may also want to get some new locks and rubber washers that hold the pod to the bracket as you will likely loose them when you pry them off.

I see that the link the previous poster listed has a vacuum diagram. If you have a hand vacuum pump, you can check all the diaphragms for leakage if you remove the glove box cardboard, lower the under dash panel, and shove the air duct out of the way. The Y34 valve unit it back in there and you can disconnect each hose and apply vacuum to each to check for leakage. If you find a leak, you can work you way down the proper line to find the problem.