Charging system road rage

McDart said
. And does anyone know what the blue wire with the red tracer at this firewall connector, the middle connector, goes to?

For this, you need to check the '72 Dodge or Plymouth Factory Service Manual like Dana mentioned. Back pages of the electrical section will be the diagrams and connector charts.

McDart said
Took the alternator and voltage regulator up to the big box auto parts store and had them both tested. They both passed.

OK. Maybe.
TJ's diagnoses approach is good. This will narrow down where the problem is.
I'll add some comments in the reposting below.

Tooljunkie said
Well first off, do you own a voltmeter and a test light?

The alternator itself is very simple to check.
One small wire has power from ignition [usually blue], other [usually green] finds its way to ground through voltage regulator.
Third, large wire makes its way to battery indirectly. [through the main splice]

So, both small wires disconnected, [turn] key on, one will light the test light. [or show battery voltage if using a meter]
Connect that wire back to alternator.
Start car with voltmeter connected to battery.
Ground other [alternator] terminal and watch voltmeter. It should increase.
Perform this and get back to us.

I wouldn't worry about the battery feed (heavy wire with fusible link) right now. It's obviously connected to the mainsplice since its running the car. That means the ammeter connections aren't the problem either.