97 Windstar - where is radiator resistor block?

Hello all,
My van runs too hot in city traffic or idling. A technician has told me that the radiator fan isn't coming on in low mode but only in high mode.
This jives with what I am seeing on the temperature gauge and hearing the fan at idle. He also told me that the 'resistor block' is the likely problem.
Where do I find and how do I troubleshoot this component?
Marc P
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