The harness is simple. If the car runs, there is likely NOT a short. What you have is three wires
1....The output wire goes from the alternator, through the bulkhead, through the ammeter, back out the bulkhead to the battery.
2...The green field wire is one wire, and only has two ends. One end is the alternator field terminal, the other end is the VR field terminal (screw terminal)
3...The blue IGN terminal (push on terminal) is branched off from the "ignition run" circuit.
So if you jumper the blue and green together, you are running full 12V to the field / brush terminal. you can hook that up and check voltage at the field terminal.