Aux fan on a 190e is turned on by either one of these two (ie both switches are connected to 1 fan, either on can turn on the fan if conditions are met - high temp or high pressure) :

Aux fan temp switch
Aircon high pressure switch (on the receiver dryer)

I have 2 aux fans in my 190e (Euro) so the temp switch turns one on and the high pressure switch turns the other one on.

Cheers.