The ignition switch is not all that difficult a job to do yourself. You can get one for about $80.00 from autohausaz, and the only tools you really need are a few end wrenches, a large socket wrench for the nut that holds the wheel on (23mm, I think), a piece of 2x4, a couple 8-10" lengths of coat hangar wire, and a VERY short flathead screwdriver.
The ignition switch assembly has three 'modules' to it: the key lock core, the wheel locking mechanism, and the switch unit itself. You have to go in through the instrument cluster, but on the older ones, it's a lot easier to get the wheel off.
1) disconnect battery (pull off Positive terminal)
2) turn ignition switch to position # 2 (steering wheel moves freely)
3) Remove center of wheel (screws in back)
You should be looking at a large nut dead center of the wheel. Make a mark to give you the exact position of the center shaft in relation to the wheel itself. The wheel is on a spline, and you don't want to get it out of alignment.
2x4 should be long enough to go from one of the spokes of the wheel to the floor of the footwell. You may want to put a small notch in the end for convenience. When you remove the nut, it may take some force to do so. What you want to do is to work against the 2x4 instead of using the wheel lock mechanism to work against. The torque required may break the wheel lock, and that is a major pain to replace, and freaking expensive, if you can find one.
4) Brace the 2x4 from a wheel spoke (left side of wheel) to the floor, put a socket on the nut, and remove it. The wheel should pull of straight toward you. Before you pull it off, remember about the alignment marks for putting it back on.
5) The instrument cluster is held in by a friction fit with a large, ribbed rubber band around it. There are two ways of getting the instrument cluster out: Use two pieces of stiff steel wire with a 1/4" bend in the ends, sliding them between the cluster and the dash, rotating 90 degrees, and pulling. Or remove the trim from under the dash, reach up from underneath, and push out. Your choice, depending on things such as environmental factors, your age, and physical capabilities.
6) The instrument cluster will only want to come out 3 or 4 inches, due to the wires and tues needed for the gauges. Now you have to reach in from above to remove: the plugs for the gauges, the speedometer cable (has to be unscrewed), and the tube for the oil pressure gauge. The last one is important to handle carefully; you must loosen the collar at the end with a 9 or 10mm wrench and unscrew. The instrument cluster should now be free to be set aside.
Note: DO NOT EVER TRY TO START THE ENGINE WITH THE INSTRUMENT CLUSTER REMOVED. This is IMPORTANT. Remember that little plastic tube for the oil pressure gauge I was talking about? That connects at the other end to a fitting on your engine in the oil lubrication system. If you try to start the engine, that little tube would be pointed straight at your chest, and large amounts of oil would come out at pressure, possibly hot. There is only a slight chance that your clothing would absorb enough oil by the time you get the engine shut down to save the upholstery.
7) The ignition swith assembly will be to the lower right side of the opening, with a large bundle of wires going to the back; remove the connector by pulling straight toward the front of the car.
8) This is the only place many people have found where M-B uses flathead screws. There are 3 of them, around the edge of the switch unit, that must be removed. On my 107, the clearances were very tight, and I had to work very carefully in a small space to get them out. Some people have suggested using a small mirror to help see everything. With those screws removed, the switch unit should come off, straight toward the front of the car, again. Note position of switch when removing, so you get the new one in the same way. There is a rib on the alignment nub sticking straight out that should be the same for the new one.
9) Install new switch, replace and tighten the screws, and replace the plug from the wiring harness.
You are pretty much done with the job, and could reassemble everything in reverse order as remove, but there are a few things you may want to do while you have the cluster out. Check the condition of the rubber band that holds the cluster in. If it's dry and hard, you might want to order a new one. Change any burned out light bulbs that you have. Check for loose wires and mechanisms. On the 107 body, the windshield wiper mechanism is right there, and there is a pivot that can work loose, causing Bad Things to happen when it starts flopping around. Look for things like loose bolts & nuts, wires or vacuum tubes hanging in bad locations, etc...
10) Reattach the connections to the back of the instrument cluster, making sure to get the speedometer cable and oil pressure tube back on correctly. The oil tube needs to be tightened with a wrench firmly, but the speedo cable should be just finger tight. The electrical connections should be designed to only go back together one way, but be careful.
11) Slide the cluster back into the opening carefully, to make sure nothing gets pinched or bent.
12) Replace steering wheel, making sure to use the alignment marks you made in step 4, and tighten nut against 2x4. There is no need to tighten this nut very hard, the likelihood of having to go back in there is pretty high. Bulb replacement, flasher or relay replacement, or cluster cleaning/repainting of needles are not uncommon.
13) Replace wheel center (I'm a bit fuzzy on this, as I have an aftermarket wheel)
14) Turn ignition switch to position 1 (off)
15) Reconnect battery.
16) Test to see if you are successful. This is a fairly "easy" job, there are a few things to watch out for, but it's not complicated or dangerous. There are few places to give you problems, and I've tried to cover them in depth.
17) Enjoy recreational beverage of choice in celebration.
Hope this helps someone, some time.
Remember: It's not just a car. It's an Adventure.