That fan uses a variable resistance bridge to drop the voltage to the fan.
When using speeds "other than fast," the voltage is dropped across a high wattage resistor. That resistance gets hot. At "LOW" it gets very hot.
I never use the fan except on high speed. Many cases of the switch burning out and a few burning up!
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW