1972 K10 charge indicator.

I am wrapping up a 99-02 5.3 swap into. 72 Chevy truck. Anyone know how to make the charge indicator (ammeter) work?

Don't know if it works bc mines not running yet (waiting on the pcm reprogram but there is the brown charge indacator light wire that goes from the alt to the pcm circuit 25, I tapped into this wire with the wire that goes to the gauge.