This is not the best help, but....

Once on a chevy truck, I had the same problem. If you can't turn the key, you can't use the little pin feature. I drilled the lock out, starting with an eighth inch bit and working my way up in size to 1/2 inch. Eventually, I got enough out so that I could manually push the little pin in and the rest of the lock came out. If you can get the dealer to show you a replacement lock, or just buy one ahead of time, you'll know how deep the thing is, so that you don't drill too deep. What I do in carpentry, when I don't want the bit to go in past a certain point, is get a small piece of wood, about 1/2 x 1/2 by whatever length will leave the proper amount of bit exposed, and drill into the middle of that. If I want to be cute or need it to be smaller, I spin the bit w/ wooden piece while touching my angle grinder with a sanding disc to the wooden part, and lathe it down to the right size.

Oh hell, this is likely as not, not real helpful. Benzes are possibly way different. But, maybe not.

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1986 300SDL, 362K
1984 300D, 138K