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One thing to look at generally is the large electrical pin block connectors through the firewall, these will be corroded all to hell unless somebody has given them some special attention. Try a brass bore brush and kerosene.
A few days ago, my heater stopped working as well as my cigarette lighters (although I think they call 'em phone chargers nowadays). After going through things step by step, I was left scratching my head. Everything checked out. Even more confusing was that they don't share the same wiring. Long story short, it turned out being the contacts on the fuse block. Just enough oxidation/corrosion was all that was needed to cause trouble. So, while the fuses were good, nothing worked. While this doesn't seem to be your problem, it just shows how important good contacts, grounds etc are. Tedster is constantly reminding us about new wires, rust free, clean metal contact etc and I absolutely agree. Even here in AZ, where we only read about rust.
Looked at the schematic for my '64 and there is only a single pin junction block connector for the blower, a couple splices, and the switch itself, later years are probably similar. It's worth looking at. Make sure the ground cable to firewall is installed, and tight. And yeah, the fuse block contacts can be shined up with a pencil eraser or whatever. Those blowers draw about 5 amps.