Ok, hopefully I can help you out.
The heater fan circuit is extremely simple.
Step #1 check the fuse. There should be one in the fuse block. Drivers side under dash. I have seen some inline fuses at the blower to, not sure if that was factory or something guys like to add in for some reason but it's worth checking for. Check the fuse both for condition and to be sure there is 12 volts at the fuse, if the truck has glass fuses then it would be a good idea to make sure there is power at both terminals of the fuse as sometimes a little corrosion build up will prevent power transfer at the relatively delicate fuse terminals.
#2. If the blower motor works on high but no other speeds it's very likely the resister. If the blower motor works on the lower speeds but not on high it is most likely the relay.
#3. If the blower motor does not work at all and the fuse(s) are good there are 3 possible issues, the switch, the blower motor or wiring.
#4. check for power at the blower motor, with the power wire disconnected you should see 12 volts on high, and progressively lower voltage as the speed is lowered.
#5. If there is voltage at the blower motor then check the ground, if it is good replace the motor.
#6. If there is no voltage at the blower, take the dash apart and check for voltage in and out of the fan switch. If there is voltage in but nothing out then the switch is bad. If there is voltage getting out of the switch but not to the blower motor then you'll need to look for damaged wiring between the fan switch and blower. It also might be worthwhile to bypass the relay regardless of the results from #2 just to double check.
#7. If there is no power getting to the switch and the fuse is good then you'll be looking for a problem in the wiring between the fuse and the fan switch.
That's about all there is to it. It might seem like a lot when your reading this over but if you just work though it one step at a time you'll find and fix the problem in no time.
