heater element
it is called a blower motor speed resistor. it is placed inside the heater box for cooling to extend the life of the resistors. depending on which wire is energized, via the blower motor switch, the speed of the fan changes, with no (or very little) resistance on high.
here is a pic and link
Blower resistor
rickf, you have a multi-meter? you could measure your stock resistor
if not, I will try to get you values in the next day or 2.