Hi azhari. The switch is located at the topside on the front of the engine. The 2 prong sensor is the switch and I hope you are referring to the one next to the valve cover. Correct me if I'm wrong.
You test the switch by pulling off the connector halfway so it's still connected. The shorten the pins with a screwdriver and the fan should engage if everything else is in order. This is how the switch works...when tempeture rises it closes the circut.
'87 MBenz 190E 2,3 8vlv