The last time I had that happening, I removed the blower motor and used jumper cables to connect it directly to the battery to test that it worked OK. It did not work OK so I went up to O'Reilly and got a new blower motor.
I believe the auto parts stores only carry the A/C blower motor, and not the heater-only blower motor. So if you have a heater-only truck it may be best to look online and see what you can find for a blower motor. There is not a lot of information about this online for some reason. Here is an old post with some info, including from GMCPaul.
http://67-72chevytrucks.com/vboard/s...ad.php?t=76855 Back when this was originally posted GMCPaul said he stocks both types of motors, A/C and heater.
The AC blower motor draws much more current at high speed compared to the heater blower, so the factory A/C wiring includes a relay to operate the fan on high, so too much current doesn't go through your fan switch and burn it up. If you have a look at the wiring for the A/C in the service manual, you'll see how they dealt with the fan wiring and relay, and the fuse is 25A instead of 10A. The electricity when the blower is on high goes from the CIG connector on the fuse panel, which is always on unswitched and unfused power, through a 12 gauge orange wire through the relay, and to the blower motor. To run an A/C blower motor in a heat-only truck you could probably add a relay to provide that amount of power to the blower when it is on high.
Here is another thread on the subject, with the service manual page that I posted with the electric diagram.
http://67-72chevytrucks.com/vboard/s...d.php?t=727010