My thermostat for the fan is on the input side of my Mesa 96 plate cooler. I did that because it is my only cooler, no oil cooler under the 911 shroud. That way the fan comes on immediately if needed. If I had two coolers, I'd put the switch on the output side of the cooler like Michael and Gordon did.
I also have an oil thermostat that loops the oil back to the engine/dry sump tank until it hits 190. Engine sump through suction pump to thermostat(cooler) back to the top of the oil tank. Bottom of tank goes to pressure pump, oil filter, then full-flow input on the case. The fan only comes on in traffic, after shutdown, or hot idling on a hot day after the engine is fully warm. I never hear the fan underway, but the temp needle never reads over 190. Things might be different in the desert...
I like Michael's idea of the light, that would be the only way I'd know the fan was running when moving above 30-40 mph.