Benzmac was right -- toasted aux fan resitor wire
On a previous thread about higher-than-normal coolant temps, Benzmac pointed out that this could be due to a broken aux fan preresistor. He also cautioned that the wires leading to the preresistor often corrode and break. That was the situation with mine. Had to have a new wire spliced in. Now the aux fans operate on low speed as they were meant to. Now I'll see if it will actually drop my coolant temps, which run about 90 degrees in 70 degree ambient temps.