I had the same problem. I took it to the indy, and he showed me how to do it. My central locking system is busted too, but this is what he did. I put the back window down. He then had me disconnect the vacuum pump in the trunk, and connect it again. It started whirring. He then used this instrument like a screwdriver with a small 90 degree bend at the end to get the threaded part of the lock up so the knob could be screwed back in. Well, he got the lock up, and quickly put the lock knob back in the hole, and screwed it in clockwise. And to be sure that I won't have to do it again, I make sure that all of my door lock knobs are tightly secured by turning them clockwise every now and then.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.