The aux fan is working in two steps. first between 80-96 C (slow speed), and above 96C (high speed).
And If you want to check the aux fan system, just look for the aux fan thermostat, it is located aprox. half way on the top left side of the engine besides the valve cover. It is the bolt with two contacts on it. Remove the socket (socket only, not the thermostat) from the thermostat at engine high temp., and the aux fan should run at high speed. It means the aux fan is okay and thermostat is not working (remove the thermostat at cold temp engine only).
Or you might check the ceramic resistor behind the left headlamp. It is for the slow speed aux fan. Using a short piece of cable try to jumper the two terminals accross the ceramic. If the fan run at slow speed, the ceramic resistor is broken (usually coroded). This happened to my 300E'88. Hopefully you can find the trouble, be carefull and have fun.