In '65, only Mustang GT's and Mustangs equipped with the Deluxe(a/k/a Pony) Interior had an actual ammeter, as opposed to an idiot light. This was an induction-loop ammeter, not the shunt-type ammeter that was used in '66 models and beyond.
The hookup is as follows... A BLACK/YELLOW wire runs from the Alternator, through a 3-wire pigtail, under the radiator bracket, along the driver-side fender apron, and ends at a Main Disconnect Harness on the firewall near the master brake cylinder. That BLACK/YELLOW wire then emerges on the inside behind the instrument panel and passes through the loop on the back side of the ammeter gauge. After passing through that loop, the BLACK/YELLOW wire plugs into (via a bullet connector) a heavy-gauge BLACK wire. That BLACK wire then continues through the firewall (through its own rubber grommet), runs along the firewall toward the passenger-side fender, and ultimately attaches to the BATTERY side of the solenoid/starter relay.
And just as an additional FYI, if that circuit loses continuity at any point, the car won't start.