When you lock/unlock from the drivers door, do the other door lock plungers also lock/unlock? When you lock from drivers door, does alarm arm? (can be verified by blinking red light on center console just to the right of emergency flasher switch.) When you unlock from drivers door does alarm disarm? (check for absence of blinking light after unlocking but prior to opening door)

There are two switches associated with the left front door lock/unlock, and alarm arm/disarm. (S47 and S62/1 on MB wiring schematic) S47 is part of the plunger mechanism and signals the vac pump to lock/unlock other doors based on plunger position. S62/1 arms/disarms the alarm and is only actuated when using the key to lock/unlock.

Without more specific symptoms, it's hard to say what the problem might be. Based on your post, it sounds like the door can be locked/unlocked but you have a problem with the alarm. From this I would conclude a bad alarm switch (S62/1). It is accessable by removing a black plastic plug on the back edge of the door just above the exterior door handle.