[HO455]

I suspect you are experiencing low voltage to the heater blower motor. When voltage drops amps go up. So if your motor draws ten amps at 12 volts it will draw 20 amps at 6 volts. If the amp draw is close to rating of the fuse before starting it will rise when the voltage drops as you start the truck and exceed the fuse rating. I would check voltage at the motor (with it on high) then compare it with battery voltage (battery voltage should be between 12 and 14 volts). There should be less than a volt difference. If it is more than one volt difference trace the circuit back checking voltage as you go until you find the voltage drop. If it is less then check the resistance from the motor power connection to ground. It should be a small amount. Then run a jumper wire from the motor case to a good ground and check again. They should be the same or very close. If they are different you have a bad ground to the motor. This should get to the problem. If not let me know and I can look into it further. As LockDoc pointed out the resistor may be the problem. The resistor gets very hot when the motor is running in slow speed. Over time this causes resistance to build up in the connector until the fuse blows. It seems like half the time someone then jams a larger fuse in and everything works until the wiring melts or the resistor burns in two. If your fuse box looks like mine did (see photo) the voltage drop (or part of it. It's quite likely there maybe more than one bad spot) could be in the fuse block.