Now about cutting into the dashboard to get at that rubber diaphragm... To me that sounds as silly as getting at the fuel pump with a cutting torch. Of course no one would do that. But this is because of common sense. Granted you won't blow yourself up by cutting a hole in the dash (at least I hope not), but it just isn't a good idea as it would leave a scar.
But what is common sense to one may not be to another. With that last post I just learned something. It sounds like that diaphram should be inspected and perhaps even replaced at the same time as the original repair, and that could only be done by removal of the dash (?). To you that's common sense, to me it wouldn't be without that knowledge. Your common sense comes from having worked on perhaps a hundred times more Benzs than I've even SEEN on the road. Please, help the rest of us avoid blunders and learn from your experience.
Thanks and happy repairing and motoring (not neccesarily in that order), Erik :~)