Is the door lock switch on a Mercedez 300SD supposed to lock and unlock all doors?
The driver's side lock unlocks and locks the other doors, gas panel, and trunk, depending on how you have set the trunk lock. If you have one you can figure out the trunk if your system is working. There are two settings.
You can NOT lock the driver's side door if the door is open. This is to prevent you from locking your key inside.
Pull out the instrument cluster so that you can gain access to the ignition switch housing. With a hammer, tap (do not beat) the housing while you turn the key. Once the key is turned, it would be a good idea to remove the tumbler unit. You can start the car by pushing the tab to the switch housing while twisting the inner piece. Order a new switch housing from PerfomanceProducts.com ($120).