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Magic!! Actually, SI alternators with their integral regulators need a small external voltage signal to turn the alternators on so that they begin to charge. In trucks manufactured w/o factory gauges, that voltage signal comes through the charge lamp. It just so happens that the mid-70's trucks powered the charge lamp through the heater blower fuse. Heater fuse blows, charging system quits charging. Same thing would happen if the charge lamp bulb blew out, except in that scenario you wouldn't know there was a problem until the engine quit cranking due to a dead battery. Trucks with factory gauges substitute a resistor wire for the charge lamp. At least with the factory voltmeter a driver would receive a consistent warning of low system voltage - assuming (s)he monitored the gauges.When the heater fuse in your truck opened, the charge lamp illuminated, because a small voltage from the regulator backfed the charge lamp and the lamp grounded through the unpowered blower circuit.Mystery solved!