Don't overthink this stuff. Keep in mind there are ONLY FOUR outputs from the ignition switch period. No magic
1...ACCESSORY...hot both in "run" and "accessory" and provides power for various switched accessories such as wipers, heater, etc. THIS DOES NOT APPEAR in the engine bay so forget it
2...IGN1 "ignition run" often is dark blue. Hot ONLY in "run" position of the key, GOES DEAD in "start." Depending on the year, this feeds things like instrument panel, warning lamps, ignition system, voltage regulator power AND SENSING and alternator field (blue), and on newer cars, smog doo dads depending on the year. Idle solenoid, electric choke, etc etc NOTE These are all just simple splices off the "main" dark blue IGN1 in the engine bay harness. No magic
3...IGN2 "bypass" usually brown, This is separate from the yellow "start" wire but both go "hot" ONLY IN start. This is the ONLY source of power for the ignition system for starting, and goes direct to the coil+ side of the ballast
4...Yellow start.......goes from key to starter solenoid, except '74 seat belt interlock goes in series with the reset box for the seat belts. Runs the start relay when the relay is grounded via NSS or clutch switch
