72 demon 340 common under dash wiring issue for no start, no power

I'd start at the bulkhead connector. There's a red wire coming thru from the battery and a black wire that runs to alternator. The connections at the bulkhead connector are the problem area due to these two wires carrying the full electrical load of the car. These end up at the ammeter, red on one side black on the other. I don't have the schematic to give you PIN numbers but they are either 10 or 12 guage wire. That's where I'd start looking. The connector is probably melted allowing the contacts to shift around. By wiggling the harness you may have moved the connectors enough to make contact.

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