If your steering wheel is lock then it is tricky. Pry out the silver washer from the ignition key lock. Then you will need to cut the ignition switch cover with a hex saw. This process means you won't be able to use your key. You will need to get a new iginition key lock and key. If your steering wheel is not lock then you can save your lock and key. you need to put the key in run. This will match the notch on the cover and the lock. You need a special tool that has two prong that goes into the two notch. You can get it from Mercedes dealer. You should be able to pry the ignition lock and the cover out of its place.
The problem could be the lock for the steering wheel. Move the steering either side while trying to turn the key.
Put the key in and move the steering wheel until the key can turn
Replace just the steering lock unit. The tumblers are old, and they need to be replaces. After replacing the lock, go to a locksmith, and have him re-key the doors to match your new ignition lock.
If the key is the cut Blade type key, it is worn.... try the spare key. If the newer key does not turn then the lock cylinder is worn... get it replaced ASAP before you get stuck somewhere. Also try rocking the steering wheel while turning the key... maybe the steering wheel lock is loading the release pin making it hard to turn the key. Never put the blade type key on a big keyring with a dozen other keys hanging from it!!! The ignition lock cylinder will be damaged resulting in the key not turning. Try a lock lubricant in the lock cylinder to help the lock mechanism turning. Good Luck...
If you have the key and it won't turn the lock cylinder try jiggling the steering wheel while turning the key. If the steering wheel lock is merely jammed that should loosen it enough to turn the key. On the other hand if you don't have the key or the lock itself is jammed you may want to replace the lock cylinder.
Jiggle the steering wheel as you turn the key. I've owned several cars in which the steering wheel lock wouldn't let the key turn if there was any pressure on the steering wheel lock.
Yes, a good mechanic should have the tools to remove a wheel lock without damage to the wheel.
The wheel should lock when you take the key out of the ignition. If it doesn't, there's something wrong.
With the key inserted into the lock.
you use the key in the door lock.
The key cylinder has failed.