Ok, don't panick!

If you've got the key cylinder to turn to the ACC position and the hole in the tumbler cylinder aligns with the hole in the ignition housing then you should be in good shape.

There are 2 methods to getting the black collar off, though they both might seem like
the same they are not.

The first method is to unfold a big paper clip and stick it in the hole, you might have to
push hard. Inside the hole there is a wedge, and when you shove a wire in the hole it
pushes this wedge down. The wedge is being pushed to the center of the tumbler, not farther down the hole.

The wedge is part of a lock mechanism that locks the black shield in place. Once the wedge is pushed down it releases the shield to turn. You might have to pad a pair
of channel locks to get a good grip to start it turning. Turn off the shield and the
tumbler will pull out easily.

The second method is different.
You can use a small jewelers screw driver inserted into the hole about 1/2". If you
lever the screw driver away from the center of the tumbler you will feel a spring
loaded resistance. This is the wedge being pushed down and releasing the shield.

Some people swear by one method or the other, but in my case I have run up
against both types of situations. My original tumbler absolutely refused to have a
wire jammed into it and release the dog. I tried several times over days using different
types of wires, and using all types of force with no success. Then I switched to the
screw driver and it worked instantly with absolutely no effort what so ever. The
replacement tumbler from MB was just the opposite. It refused to open for the screw
driver. I couldn't make it work for anything. I took it back to MB as defective and
the parts guy took a paper clip and stuck it in and worked the first time! Screwdriver
still failed, paper clip worked.

Good luck!

__________________
Will W - 1984 300SD federal