A perplexing problem solved.  The blower fan operated normally at high speed, but failed to operate at the "slow" speed position.  The switch seemed to be in good shape, and spraying it with contact cleaner was of no help.  Apparently the wire-wound resistor attached to the switch, and employed in the "slow" position, changed value upwards as a result of the heat generated by the constant load over the years and now did not allow sufficient voltage to run the fan motor at all. 
The solution:  Adding, in parallel to the existing resistor, a 1/2 ohm to 1 ohm, (preferably a 20 watt wire-wound) resistor drops the total resistance enough to provide sufficient juice to the motor to run at a reasonable and consistent reduced speed in the "slow" position.  I soldered 12ga leads to the buffed-clean resistor terminals on the switch (leaving the original resistor connected), and connected the wires to the extra resistor which I anchored elsewhere via the support provided with the resistor.  Insulate any bare connections.  Since neither resistor produces very much heat, the extra unit can be positioned anywhere out of the way.