You sound as if you are not in the USA
The 300SL was not a US spec car. I am going to give you information that would be relevent to US spec cars and may or maynot work on your car.
The central locking pump recieves signals from the vac lock actuators in the doors. they have a three pin electrical connector on them. In the rocker panel area just ahead of the door opening is a flat three pin connector that has been known to become corroded and cause a short that will drive the CL system nuts. The best thing that you can do is get a wiring diagham and isolate each part of this circuit untill the fault is laocated and repair. Good Luck and have fun.