unusual central lock problem

I have a 1987 300E. The central locking system does not lock or unlock when I am using the actuator on the driver's side door. However, the entire system works just fine (including the lock on the driver's door) when it is activated from the passenger side door or from the trunk. Therefore, I think this is not a vacuum line problem, but an electrical problem. I tested the electrical function of the driver's door actuator, and it's voltage supply - both seem OK. Where do I go from here? Your technical wisdom here would be greatly appreciated.