Replace the lockset, take your title to the MB dealership and they will make you a new one which will also come with a new key that will match all your locks and you old key will still work in it. If you are a MBCA member you'll get a 10-20% discount.
If you think the steering lock is bad also, it only engages when your steering wheel is centered, so, cut your wheels hard to one side or the other when you park and before you shut off the motor ane the lock won't engage.

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Houston, Texas
1984 300CD 235K miles, Elvira, one owner
1987 300TD 180K miles
1974 TR6 78K miles Sarah Jane, one owner
OBK #27