If you look at the schematic the wire that goes to the ballast resistor is on the 12v side so it could go anywere that had an ignition switched 12v source.
Hmmmm. So what your saying is basically what Dave is saying. Just because the schematic shows it don't believe it is exactly as shown. Unfortunately I haven't worked on a 70'ish charging system since the 80's and I never really ripped open a wiring harness to see how they ran the wires anyway.