The FIRST thing to do is try and identify what brand the harness is in the car. This is because whoever manufactured it has (varying) documentation on that harness. The wires are generally either labeled or numbered.
OP does the car still have the bulkhead connector plug or has that been eliminated into the cable feeding "straight through?" This is because you will need the pin numbers to figure out where they go
Some of what you "need to find."
There are only so many outputs from the ignition switch. They are ACCESSORY, hot with key in both run or ACC
"Run" (IGN1) which is only hot in the "run" position. This feeds stuff like gauges, warning lamps (brake and oil) and comes out through the bulkhead and feeds ignition system, alternator field, and regulator power
The coil bypass circuit, "IGN2" in the diagrams. This is hot ONLY in start just like the start circuit, but is isolated with a separate switch contact. It goes out through the bulkhead and connects to the coil + side of the ballast resistor (if you have one) so the ignition system gets full battery during cranking
The "start" wire. This is also only hot in start, goes out through the bulkhead, and triggers the start relay (one of the small flag/ spade terminals on the relay) The other flag terminal either goes down the firewall to the center of the neutral safety switch (automatic) or through the firewalll to the clutch safety switch (stick shift.) Either of those grounds the relay coil and allows it to function
