My Benz is a 300D 1983. If yours is anything like mine, the fan is most directly connected to the Air Conditioning system. When your AC is on, the fan runs all the time. 100C sounds like it's a little hot to me though! Your thermoswitch will be on top of the AC receiver/dryer. there are 2 sets of wires, one for the compressor....making sure there is refrigerant pressurised in the system. Those are connected to a small silver looking can on the rectangular part on the top of the receiver dryer. the thermoswitch is located on the same rectangle, but off to one side of it. Jumping those two wires together will cause your fan to run all the time. If it still operates intermittently, then you have problems with the fan motor and it could need to be replaced or rebuilt.