Here's some reading on the 3-pole temp switch - Coolant temp sensor 1987 300D

There's a low speed fan switch in the refrigerant circuit and a high speed fan switch in the coolant circuit. It's difficult to test the low speed fan switch since it's impractical to measure refrigerant temperature - I don't know if an IR temperature reading of the exterior of the low speed switch is representative of refrigerant temperature, and the switch is exposed to the refrigerant system - you can't replace it without discharging the refrigerant.

Sixto
87 300D^2