I have no experience with the W140, but I think is pretty much engineered like the other models...
Pry out the door rubber near the doorlock/handle and partly unscrew the "imbuss" (I don't know how they are called, but it has the 6-edges inside the head) bolt that you can see (using a flashlight). Don't turn it out all the way, but just turn it a little until you can shove the door lock far enough to pull it out. The door lock has to be pushed away from the handle after which it can be taken out (if the bold is unscrewed enough).
You probably don't need to turn the key anymore to get the lock out (normally you would have to turn it to get out a pin that is stuck inside the door locking mechanism itself) after which you can see what broke off.
If you need the pin only, you might end up going to a car recycler because I think that mercedes only sells complete locks. If you need to order a new lock, you will have to go to the dealer to get a new one by VIN# or chassis# (fitting your original key).

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)