DHow does the amp meter in a 66C20 work?

[StevenG]

I have a parasitic drain to the battery when the truck is off. When I connect a test light between the Neg terminal and the neg cable it lights. When I disconnect the blue and white wire to the alternator it goes off. If I disconnect the voltage Reg it will go off. When I test the voltage on the brown wire to the VR I get 12.6 V when the ignition key is in the off position. The brown wire is from the amp gauge in the dash cluster. The ignition key switch is new. The alt is new, the VR is new. The truck has all the wiring replaced under the dash, under the hood and to the rear lights as well with American auto wire harness. Their products are second to none. The amp gauge never moves, the needle stays in the center remove the battery no movement. Start the truck and the alternator sends 13.6v to the battery but the needle doesn’t move. It just stays in the middle. So how does the amp gauge know how many amps are being drawn? Does it have a shunt or sorting to test the load? Thanks in advance for your comments.
Steve