I know this is covered multiple times but I thought a picture may be helpful to others (even being a poor quality digital camera picture).

I was experiencing the early symptoms of ignition lock trouble - difficulty in inserting the key. Being cheap/frugal I decided to take the $30 solution of a new lock cylinder with a new key vs getting replacement cylinder keyed from Mercedes for $100.

The lock cylinder is held in place by the hardened black cover in the picture. Also visible is the key-way in the inside of the cover that cylinder keys into. The hole in the front of the key-slot is where you insert your "tool" The manual shows a 1.25mm dia rod (I'm going from memory here) or about .050" you insert your tool - paper clip, drill rod, small screwdriver etc and this pulls down the "key" in the cylinder that engages the cover. The hole only lines up with the key in position 1.
That is critical - you must be in position 1
Once this is in place the hardened cover can be unscrewed. In my case I managed to get it out with a combination of plain old needle nose pliers and some old bicycle inner tube to get a better grip (I did mess up the black paint some but I can fix that before putting it back together). If I were to do it again I would probably make a special tool on the lathe similar to what they show in the manual.

Anyway once out you can see the problem - the lock cylinder was coming apart. The front face of the cylinder (round piece in the bottom of the photo) is attached to the keyed part (silver barrel to the right of the black cover) by a small web of casting - this had fractured (I removed the internal part of the cylinder by inserting the key to get the "pins" lined up so it could be removed)



Once out its easy enough to slide the new lock in place. It really helps to see the replacement lock so you know what needs to happen.