The tumbler is likely the fault with this mileage. My switch blade FOB key wouldn't turn the tumbler without a lot of finesse and effort. Got a new flat key and the tumbler worked well for about two weeks, but the same problem started and the tumbler wouldn't turn w/o a lot of effort.

Took the tumbler and switch out from the bottom as I wanted to get a good look at the switch and all the connecting parts. The problem was the tumbler and you could see that the fins didn't drop down all the way into the cylinder to allow the tumbler to turn, when you inserted the key.

If you can get the mercedes tool to take off the black ring, then the tumbler will come out and you can put a new one in from the front. If you are handy and have the time, you can take it out from the bottom. I posted my experience on this before and there is a picture tutorial. Or you can pay the pros/stealers to do it for you with their Mercedes tool. They should be able to easly do it in 20 minutes or less.

I hope this helps,

Pabster