'66 Valiant Weirdness

Do I need to make an adjustment to the Voltage Regulator?
Probably not but since its off now might as well do the easy checks.
Should I start checking grounds?
Yes.
And then the power feed and charge connections.
Here's the thing to understand.
There are two power sources, right?
But the one can only supply power at 12.8 Volts in the best of situations.
The other can supply power at 14 volts give or take a little.
So between the two possibilites, electricity always flows from the higher voltage source, as long as it has enough power and the resistance is about equal.

The ammeter is in line with the battery.
It only shows if the alternator is recharging the battery or if the battery is discharging.
This would be the current flow of a normal recharge after starting.

basic-charge-circuit-charging-animated-gif-gif-gif.gif

After the battery gets recharged, the flow through that line is essentially zero.
upload_2019-9-27_20-48-1-png-png.png

But if there is a loose connection on the alternator output line, then the battery will fill in and the ammeter will show discharge.
I thought I saw that a couple times in one of your vids, but I might be mistaken.