Thanks for the advice. You're right - I live in the UK and the 300SL is therefore European spec. The US CL system you describe sounds identical to mine - I guess it would appear on your model 560SL which was out about the same time I think (late '80's).
Could you be more specific about the location of and access to the rocker panel where the 3 pin connector is? Is it in the body of the car close to the door hinges? How do I get at it?