[VetteVet]

It's like Ray and WB said, you have to run the alternator output wire to the main junction of the power wires.
What the gauge does is compare the alternator output to the battery state of charge. The battery side of the gauge is connected to the positive battery terminal with the fusible link at the right fender junction.Then the other wire that is on the fender junction runs to the main power junction in the harness. This wire is called the SHUNT, it also connects the alternator output wire with the battery. It also carries battery voltage to the truck circuit when the key is off and the engine isn't running.

The alternator side wire of the gauge is connected to the main junction in the harness with the shunt and the voltage sensing wire. Then a fourth wire carries both of these power sources to the cab for truck power.

A good way to picture it is to imagine that your body is the ammeter and with your right arm straight forward your right hand will be the right fender junction where the battery connects.

Then with your left arm straight forward, your left hand is the main harness junction where the alternator output wire connects.

Now if you connect the tips of your index fingers, the become the SHUNT to connect the battery and the alternator. Your arms are the connections to the gauge and they measure the voltage differential between the battery and the alternator and by being connected to the battery gauge, it can show charge or discharge depending on whether the alternator is charging or not.

Here's a diagram to show all this.