This seems to be a common problem on mid 70s Dodges...is there a common problem/cure. I've already changed the blower motor. This happens to be on a 74 truck but I've seen it occur on others also? 20amp fuse gets extremely hot but doesn't blow out?

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1968 Plymouth GTX
1974 Dodge P/U Long Bed Stepside 318
2019 Ram 2500 6.4, auto, 4WD