I have also found in some cases it is just the key itself that is worn out. I know it sounds simple but sometimes the simplest things cause the most problems. Making a copy from the worn out one is not usually a good idea but if you have another key it's worth a try...or you can get one from an MB dealer. (assuming that it's the original ignition)

__________________
1982 300D Turbo "Helga"
380,000 miles