High volts

Sounds like (apart from my misunderstanding) that you are pretty much running this down. The wire itself---apart from corrosion right at the terminal, is usually not a factor, unless it has been (example) slammed/ pinched/ etc broken in such a way that many of the strands internally were broken. MOSTLY connectors is what you need to concentrate on.

Somehow I didn't catch that you had .3 right in the switch, as I wasn't sure you were probing the switch side or the dash harness side of the connector.

You're getting there.

I should have mentioned this some time ago. Still another approach is to simply use the IGN switch to key a relay. I "did mine" all in one swoop--found a relay box out of a junker, which gave me enough relays for EFI, pump, hi/lo beams, and a relay for ignition/ charging. And one left over for security, ?? other. So all the IGN switch does nowadays is key relays and provide direct power to the cluster/ gauges.

Mine originally had more than a volt drop total from battery to ignition.

This box was out of a 90s Voyager, I think. Mounted right on the top of the inner apron to the rear of the battery. I did have to do some internal re-wiring.