I just had the experience of replacing the switch in my 190E because the key locked up and wouldn't turn to position 1 for me to replace the tumbler. After some research I find that this is a common problem. I have created this article to carry my weight around the Forum. I used many posts from many forums to gain the courage to tackle this. So get set, this is what it will look like shortly...
Go to Mercedes Benz dealer and order VIN specific tumbler and key. I had this done in Bedford Texas and it cost me $120. Normally 2-3 days wait, but the keys are made in Fort Worth Texas (a stones throw from my job, ironically) and it was waiting the next day. The price for all pieces was about $300, key/tumbler - L-shaped switch housing - Black tumbler cover - Buzzer/Dinger Switch (optional, I think you can cut the slots that keep it off and remove it, be careful of the fragile white button underneath, you cant slide it off in Position 0; you can also just leave it off and not have a dinger/buzzer)
Remove negative battery cable and insulate.
Remove 2 plastic covers at left and right of steering column to expose 3 Phillips screw heads. Remove screws. Underneath, there are 1-2 more fasteners. Remove the under dash panel
Using 10mm socket and extension remove two nuts and remove knee bolster (that oval shaped black thing)
At left side of air conditioning box, there is a black square vent and hose. Remove the square vent part and pull the hose out of the way (your hand will have to go way up there, clear some room, cut zip-ties and do whatever it takes)
Using a 4mm Allen Hex Socket (I think) insert it into the back of the steering wheel where you see 2 small holes, the airbag will come loose. Be extremely careful with the Airbag!
Don't drop it or allow anything at all to touch the small connector on the airbag. Keep in a very safe location during this job. Unscrew those 2 bolts and support the airbag. Remove the small red connector. Secure the Airbag.
Using a 10mm allen socket and extension and 1/2 inch breaker bar. Remove the bolt holding the steering wheel. After the bolt is gone, the wheel will easily pull off of the splines.
Wipe off the back of the steering wheel and the brush contacts for the Horn and Airbag.
Remove the 3 small #1 Phillips screws (don't lose them)
Remove the aluminum plate that the 3 small screws were holding. (I pulled the green harness through an oval hole and unplugged the red connector up under the dash)
Remove the 3 #2 Phillips screws holding the combination switch to the column flange. (one screw is longer, it goes at about 12 o'clock)
Separate the upper and lower halves of the steering column cover. (I couldn't do this, it looked as if someone glued mine together, so I just wiggled and wobbled until the whole thing came loose and it all laid down on the floor)
Pull cluster out by the edges until it gives resistance, then push back in slightly. Lying with your back on the drivers' floorboard, reach up through the cleared spot from step #5 and unscrew the speedometer cable with your phalanges. (fingers, I used my thumb, I got big hands)
Pull cluster out and support (you can just leave it dangle and do your work or get it out of the way and disconnect all the plugs, they are all different plugs, so there is no confusion. The small indicator lights have numbers on the wires that coincide with the number of the socket on the cluster.
Remove the small trim piece covering the lock tumbler.
Insert a 10mm socket from the opening where the cluster was, this will allow you to unscrew clamp's pinch bolt that is holding key switch to the tube. Loosen bolt. Locate 2 electrical connectors that are on a rectangular black switch on the switch assembly. This is for the key reminder buzzer/dinger.
On steering column tube, locate metal dowel inside a hole. (This dowel should release at position 1, but not today...at position 0)
Drill that dowel into nothing, it will spin, but just keep at it. Use a sharp bit and go for it. I wasn't too concerned with the metal tube, so I just tore it up.
The switch assembly should now be free to move around a bit. The electrical connector on the back won't let go unless you're at #1. I did a fancy thing that I hope works for you: I took an orbital sander (the same sander I used to rattle the tumblers into position before I ever started this project) and sanded the switch until I could see the 3 screws that hold the electric part, back out of the holes! Now, I had to loosen them a bit with a chisel in between the 2 sections, but it worked. After the screws all backed out. I was able to separate them and pull the switch part out. It's tight but it will go. Make it happen.
Pick up the switch from MB and have them assemble it for you. They did it, no questions. Also buy the buzzer/dinger switch, unless you can get it off the old one. Most folks bust it trying that, MB said.
Install the electrical switch section with the 3 screws on the new switch.
Install the new switch assembly. (Easier typed than done) I struggled here. With the Key in and position #1 to hold the lock dowel down, there was hardly enough room for the switch, let alone the switch and brand new key. So I got a friend to pull the dash back toward me while I maneuvered it in place. It took some grunting and I am sure there is a better way. I just didn't know one. I actually cracked part of the dash beside the dual air vent while pulling on it. After the switch is in, hookup the big connector with the key in #1. Tighten the clamp and give it a whirl. (don't hook up the battery yet! Just work the mechanism and the steering lock)
Hookup the dinger switch.
Install the cluster (don't forget Speedo cable)
Install the combo switch and column trim halves (do this carefully and inspect it after the aluminum plate with the brush contacts on it. I put mine on and it was crooked. The plate should be absolutely flat in comparison with the steering shaft machined surface. If it's crooked, remove the small plate, remove the combo switch and recenter and tighten again. (I had to do it 3 times and the 3rd time I used a cordless screwdriver and feathered the screws to lock it down properly)
Install Aluminum brush plate and red connector under dash.
Install steering wheel and airbag (use caution with airbag) (torque steering wheel with someone pushing the wheel against your torque) I admit I don't know what the value is.
Reattach black square vent and hose (under dash area)
Zip tie any wires up that need securing
Install Knee Bolster
Install Under dash cover
Install plastic trim covering screws and key tumbler trim piece
33) Be sure that Airbag proximity is clear and safe, cross fingers.
Hookup Negative Battery Cable. (hold breath)
Take it for a spin
I might have missed a small thing or two. I did it a week ago and it's not so fresh in my mind. Look for more DIYs from me, I really enjoy this Forum, it's the best Benz Forum I have found, I think I've tried them all.