1991 MB 300e

Recently my central locking system has partially stopped working.

I can lock & unlock the driver door or the passenger door with a key, but all other doors, trunk and gas flap do not respond.

I can lock & unlock each door from inside the car by pushing / pulling the lock button, but all other doors, trunk and gas flap do not respond.

I can lock & unlock everything like normal from the trunk key only.

The pump (under the rear seat) seems to be functioning fine and the action is quick and precise when using the trunk key. I cannot hear any air hiss at any of the four doors to indicate an obvious air leak there. I can see the other three lock buttons move almost imperceptibly when any of the four lock buttons are pushed or pulled from inside the car, but they do not raise or lower.

My first reaction is start pulling off door skins, but I would sure appreciate any advice as to the most logical starting point.

Thanks everyone