Accessory fan, (AC, heat etc.)

The 'do-hicky' (I'll call it a regulator) that goes between my cars dash mounted fan switch and the fan motor which controls the flow of air into the car (AC or heat, or defrost) is fried. First it ran fine, then only on high, then it 'surged' from nothing to high and then it just stopped. I found a parts guy who knew which part, but I can't find anyone who knows where it goes.
It 'might be' under the wiper cowl. Or it 'might be' under the dash.
Any help you could proide would be most welcome. Thank You Very Much.

Timothy Florida