That shunt is a resistor wire on these trucks. The actual wire itself is the resistor, same for the coil power for the ignition. I'd bet its the new regulator and here's why: Ford lists two different part numbers, one with idiot light and one with an amp gauge. The wiring is different in both cases. All the aftermarket regulators replace both styles.

Before I did the 3g swap on my fairlane I had the same thing happen, old mechanical regulator started overcharging to 18 volts. The part store replacement caused the alt light to be dim with the engine off.