Ford Duratec engined trads including 3.7 Roadster from at least as early as 2004 to end of production used two relays to independantly provide high and low speed fan cooling assistance to radiator when needed. Each had a common +12 volt power feed and common +12 volts to coil. A Ford high power resistor plug was included in the circuit after the low speed fan relay wired to give lower voltage to cooling fan when its relay was triggered on coolant temperature by a ground from the ECU. If coolant temperature rose higher the matching high speed fan relay was triggered by an independant ground from the ECU and its output fed direct to cooling fan, effectively bypassing the resistor. I believe the low speed one remained triggered when coolant temp rose high enough to trigger high speed relay so you're never without some fan cooling assistance when needed if high speed relay failed to trigger. Each relay must be of type with diode protection to remove back emf spikes detrimental to ECU.
