MOST MB's actuate the aux fan based on high side pressure or temperature. I know that on an 88 300 E there is a red switch at the receiver/dryer for this.

This is very typical because the fan is used to bring down high side pressure to prevent a/c system strain. This is why you will often here the aux fan still running after you have turned off the engine, locked the car and are walking away. The high side pressure has not had time to equalize.