Simple fuse box fix?

68gtxman said

............... I tracked it down to a new positive battery cable that had a factory defect. The crimped lug on the end to the solenoid wasn't crimped enough to carry the starting current.

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THIS IS a very good point. I got my brain entrenched in what the OP listed as results, and it just might be........

Try probing the "big stud" on the starter relay and see if that also goes dead, when headlights or key is activated............