blower motor resistor

If the blower motor has been replaced, or it's old and failing, it might be drawing too much power. Not enough to blow the fuse, but enough to burn up the resistor block. You can also try a Motorcraft resistor block instead of an aftermarket one. If you haven't done so already, replace the plug (pigtail) that plugs into the resistor block. That might be the real problem. Jim

I agree , junk parts have been a problem for years , Go to the ford dealer , bite the bullet and pay for a quality part

See how many amps the blower motor is drawing as that is the usual cause of resistor block failures, the other is bad grounding circuits.