MikeE,
I did a search of this site on the subject of resistors in the heater blower motor circuit.
They all indicate the resistor gets VERY hot, but just like the Timex, keeps on a tickin!
It must be the Heavy Duty rated resistor, the one sold for heaters, not gauges! One member mentioned mounting his in the heater housing so the blower would help cool it, plus it was out of the way of incidental contact with a stray hand or child's finger.
I think the motor draws 6 amps + at 6 volts, so you have quite a bit of energy to dissipate in the form of heat.
While you're poking around under the dash, keep a look out for the OTHER wire...it's in there somewhere!
Stuart
