The tan alternator power wire goes through the large plastic plug on the firewall, next to where the steering column goes through. There are no other wires on the ammeter. Just the tan going in and a red wire of similar size coming out to the starter relay.
Factory heavy duty should be a rubber grommet, and the hole should be in one of the locations the factory dimpled for that purpose.
Unfortunately not many examples of heavy duty alternator wiring have been shared or spotted. I think the link above is the only '73 on FABO with this ptional wiring photographed.
If your car had this option, but without rear defrost grid, then perhaps the power is getting to the main splice through the standard alternator output wire.