on my 88 190e 2.3, I was able to engage the fan by pulling off the harness from the sensor on the top of the block, closest to the front of the car. My sensor has a red base. Then I held a staple in a pair of needle nose pliers and pushed the staple into the harness I had pulled off the sensor. Then the fan came on. I ended up needing a new sensor. Now the fan works fine.
BTW, this is for the fan driven by the electro magnetic clutch. Not the aux. electric fan.
If I remember correctly, i think I pulled off the harness of the sensor behind this one and the aux. fan came on automatically. Worth a try.