Dual Field Alternator Conversion - New Electrical Gremlins!
No. There is no other circuit that should be "feeding back" like that.
Sounds to me like there is something crossed up in that harness. If this were an older harness that could have been damaged by a high current short, I'd suspect "meltage" in the harness, IE a couple of wires melted together.
I cannot think, really what might be doing this. You might have to "rig" in order to separate the general area of the problem, something like this:
Remove all the engine bay connectors from the bulkhead connector so that it's "blank."
But now, you cannot get power into the car. You also need to protect against "screw ups" so you don't burn things up. You have an old headlight? How about a tail / stop lamp socket? What you want is a heavy wattage bulb.
"Rig a heavy bulb" such as a stop or backup lamp or headlight with alligator clip leads.
Figure out which bulkhead connector terminal is the RED fuse link That is, power feeding into the interior. Hook one end of your lamp to that, and the other end of the lamp to battery hot. This will feed "protected" power into the car interior.
Double check that items internally in the car operate. headlight switch should make the dash lamps work, and dome light should work.
With the ignition switch connector engaged, turn the key to run and to off and verify that the dark blue "run" wire goes on and off.
If that checks out, get a wiring diagram and do a wire by wire continuity check on the engine bay ignition harness wiring. Start with the dark blue "run" circuit. It should feed off to the ballast connector, and the VR. Check for continuity from that wire to all other wires in the firewall connector.