Charging system road rage

So it seems the charging system in my 72 swinger is not charging. I've barely touched the surface on tracking down the issue and I'm already pulling my hair out. I hate electrical.
Dash gauge shows discharge when running (see pic). Obviously because its running but not charging, yes?
With engine running, I disconnected the negative battery cable.car died.
Took the alternator and voltage regulator up to the big box auto parts store and had them both tested. They both passed.
I repaired the connector on the power lead to the alternator and made sure I put me field and statter connectors on the right tabs.
Started up the car looked at the gauge is still show discharge. Disconnected the negative cable car died.

Pulling my hair out as I do not know where to go from here, where to look for the problem.The wires at the firewall connector for the alternator and engine harness look not so great and I like to replace this harness but I don't know where to go from here. And does anyone know what the blue wire with the red tracer at this firewall connector, the middle connector, goes to?
I would really, really like to start driving this car now that I have some of the small safety checks/repairs done.

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