Sorry, firemediceric, the info in my post is for the 1987 300D (original post topic), not your 1981 300SD. That 3-prong switch doesn't exist on your car, 
AFAIK. And, I think on your engine, the thermostat is at the upper radiator hose, not the lower. The 124/603 and 126/617 are TOTALLY different systems.