I have checked the fuses inside relays and maxi fuse outside and they are all good. I get no power to the switch but I can jumper to the motor and it does spin.
RepairPal Certified shops can diagnose and repair your 1991 GMC C2500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
If we're talking the heater and AC blower motor in the truck, look for your blower motor resistor - you will probably find a melted wiring connector at it, or the resistor may be faulty.