My bad...I’m talking about the switched 12 volts not the big wire to power the box.
If I’m remembering correctly, the big wire powers the box and the 12 volt switched wire is what turns the box off with the key.
I wired this same box to a 67 Firebird about 4 years ago and it was much easier. I just replaced the factory resistor wire with a regular wire and hooked it to the box.