I just thougth I'd post this little gem that I dug up from the archives of a forum when my ignition key woudn't turn about a year ago. I thought I was going to have to remove the steering column.

This was the solution I found:

Get hold of a finishing (vibrating) sander (I got one from Lowes). Clamp your key to the edge of the sander using its provided clamp for the sand paper.
Then insert the key and plug in the sander. The key then turns right away.

Having done that, I was able to remove the tumbler fairly easily (from position "0", you have to put a pin (paper clip) down a small hole and then unscrew the retaining nut). I then figured out that putting a piece of the right-sized metal at bottom of the hole holding the mechanism from locking the steering disabled the steering lock (I used a rolled up piece of hose clamp). I also broke off the metal piece on the outside of the tumbler/lock assembly that prevents unscrewing the retainer unless it is unlocked, as an additional safeguard. Now it of course isn't as secure but who knows that, and my old rust bucket diesel isn't exactly a prime target. And it's been fine for a year now.

I can't now remember for sure what was the cause of the problem, but it was not the lock itself but somethign to do with the steering lock mechanism at the bottom of the hole. But I didn't want to take the chance of getting stuck again and maybe the sander method wouldn't work next time.

I'd guess that it is the same cause in most cases like this in the old Mercs. The proper fix I suppose is to fix that steering lock mechanism somehow, but that may involve removing the steering column. If you don't want to damage your key tumbler like I did, you can just leave it out until you fix the mechanism, and turn the ignition with a suitable screwdriver.

thanks to whoever figured out the sander method and posted it years ago, and I'm just resurrecting it here.
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ECIGARETTES