There are a couple of things to mention here. First of all, if you're having trouble with the key portion of the switch, it could be the steering lock. If the steering lock ever completely locks up, the complete steering column must be removed and the old lock drilled out.
There are two locks, the ignition key switch, which actuates the steering column lock.
One other thing to mention, use a steel key, not an old brass key, the brass particles fall into the tumblers and cause problems. You can give your dealer the VIN of your car and they can get you a steel key, and/or a replacement lock cylinder keyed the same as the original.
The procedure for removing the key is in any service manual, one of which I am not near right now, or I would give it to you here.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in