I had the same problem after the temp sensor went bad on the 190e. To stop the fan I found the fan connector near the AC receiver dryer bottle and disconnected it there.
I replace the temp senor, reconnected the Aux fans and the problem went away.
If you are replacing the sensor, be sure to get the one rated for your car. On close inspection the thermo rating is labled in small print on the metal of the sensor.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)