An Electrical Nightmare

On a '72, the blue RUN wires go to ballast resistor, the regulator, and the alternator field. The connection at the regulator is so to tell it system voltage. its just a just a measuring point. Power to the alternator field from the blue wire flows through the windings - and out the green wire to the regulator. The regulator controls the connection to ground. (If the green wire is grounded max current flows).
With an electronic box, a second connection is made on the coil side of the ballast resistor.
Alternator's black wire is power output. It is always hot. It will be at battery voltage when the alternator is not producing power.
Some illustrations of power flow in Understanding Charging Systems with Ammeter