Burnt Wire

Some of what you are doing:
Since the short seems to "happen" when you have the key in "start" than we need to determine "where all" power goes when in start

The key switch gets UNFUSED battery input form the ammeter circuit, live at all times

outputs from the key:

1...With the key in "run" the IGN1 "run" usually dark blue is live as well as the ACCESSORY buss below
2...With the key in ACCESSORY only the ACC buss is live. This feeds the buss in the fuse panel that fuses feed radio, heater, wipers, etc

With the key in "start" the IGN1 "run" circuit and the ACC buss ARE DEAD

ONLY the following 2 circuits are live:

A....The yellow "start" wire feeds out through the bukhead and goes to one "push on" starter relay coil terminal With the clutch depressed, or the trans in P or N the neutral switcn grounds the other relay coil terminal so the engine cranks
B..Since IGN1 "run" is now dead we need power to ignition. The IGN2 brown goes from the key through the bulkhead, to the coil+ side of the ballast resistor.

SO THERE IS NOT MUCH "stuff" powered in "start."

What else DOES have power is all the stuff in the car that is "hot" at all times, like dome lights, head lights/ tail lights etc etc anything wired to the "hot" buss in the fuse panel. BUT THESE COMPONENTS are not affected by the "start" position