These ign switches suck so bad. When you get done drilling or vibrating the thing apart, don't put the working key lock or lock pin back in . I took the lock part apart but left the cylinder to accept the key. 3 times my 190d broke its switch, it has a link between the locking portion of the assembly and the electrical switch in the back. Its' a 3/8 long pot metal tube with ears that shears off. Easily. oh, I remember some thing about the dash comes out for access. Position 2 in key rotation allows a stiff wire(2?) to be poked into it and push a snapring or such out and allow the lock cylinder to be pulled out. out The electrical part on the back has screws I think. Modify the switch asembly to look like the key is needed but not actually.And keep your mouth shut about it. She won't know, and she needs the key to lock it.it is not safe to replace the pin lock or the key lock because of the failure rate. It ain't safe for someone to be marooned over this, so.
To keep my 84 190 in service I did these things, after the 3rd fail. My 126 has a sticky switch lock and it's getting the same switcectomy when I break it out this spring.
I really do like my mercedes,very pleasant to drive. good luck.