They are mounted up front of the radiator. They come on when temps get hot. They are two speed, so they will run at low speed after a moderate drive and high speed if it's a very hot day, or the engine was under a heavy load for the trip.
You have a temperature sensor mounted either on your coolant pump or your cylinder head that when disconnected, the circuit operates in a "closed-loop" mode. The fans will run at high speed when you pop the connector off.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car