Found my dads original 1971 duster!

Mattax said
H',m interesting.

A loose connection is my only guess right now since when you directly connected the battery wires the car started and ran.
Since we're not confident of the where the break was, it wouldn't hurt to gently tug and push on the wires at the bulkhead connected to see the terminals aren't loose. The terminals have barbs on them sometimes they don't catch or hold.

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Thanks again for all of the help. The builder was supposed to put all new parts in the car. Then asked me 7 months in if I wanted to buy a new wiring harness for it. I asked him what normally happens and he said that he always just uses the old wiring. In your opinion should I be ordering something like this? Or just use mine if I can find no visible damage etc?