Wiring a three wire alternator up with factory ammeter gauge

[Harrison.]

I know the B/W wire coming from the firewall connection goes to the ammeter gauge, it appears a 12 gauge B/W wire would have gone to it on the engine side of the connector? My truck did not come with factory gauges and a generator, I am going to run a 3 wire alternator and putting factory gauges in. How do I need to wire it in? Will any hot lead work for the ammeter?