After a chilling winter without heat, eventually ended up putting in a another fan switch, blower motor and blower resistor. Worked for a week or so, then yesterday hit the hi on the fan and pop, there goes the fuse.
Put another fuse in without the blower motor hooked up, hit the hi on the fan and pop goes another fuse. OK, not the Blower motor. The fuse didn't pop on the first two fan positions.
Looks like the thermal switch is shot on the blower resistor, but it should still work in Hi position.
I think I'll try replacing the fan switch and blower resistor and see if that solves the problem. Hopefully the fan switch is where it's shorting out.
I've had this trouble for at least 10 years or so, hopefully some day I'll get the definitive solution.
The lemon book wasn't kidding when they said my electrical system is poor.
88 Jeep Comanche.