'71 Dart/Demon engine wiring harness question (ballast resistor wires)

Don't know about your question, but it is simply a branch of the "key side" of the ballast, AKA the "ignition 1" or "ignition run" line. Depending on year, there could be several loads on that line, generally, ignition system, alternator field (70/ later), VR IGN terminal, electric choke if used, a one or a two smog/ emissions devices in later years, and you already know, the idle solenoid. That line IS NOT FUSE, and is commonly prone to voltage drop, which causes OVER charging

It is not a bad idea to cut that line close to where it comes out of the bulkhead connector, and connect the bulkhead connector end to trigger a "Bosch style" relay, then connect the engine bay end to the switched load contact. Feed the contacts with a good heavy fused line AKA off the "big stud" on the starter relay