My Hotwire Auto "Hotrod" EFI harness hooked into my '68 A-Body harness with 4 wires. One was a ground and two hooked to the combined 12v supply of Start 1 and Start 2. I forget where the 4th one attached, but it was super simple and very neat.
I would think battery +. We build the same harness and they require Batt, ignition(start/run)+ and ground. Overdrive trans requires an over drive off wire also. Super simple.