I've got 2 aux fans on Babybenz.
One is connected to the aircon high pressure switch (it works only with the aircon ON) and the other is connected to an engine temperature switch (is turned on by the engine temp at 87 deg C).
If you've got one aux fan (like my kid brother's 190e), it should be connected in parallel with the engine temp switch as well as the aircon high pressure switch. Meaning, the aux fan can be turned on by either one of these switches closing.
Babybenz 1991 190e 1.8