Electric Choke

There is only one source, period. that is "ignition run" or IGN1, usually dark blue, and comes from the ignition switch

^^EDIT^^ I was typing same time LOL

Also you could use the output of that relay to feed the VR. Often that blue "run" line suffers VOLTAGE DROP in these old girls. This causes OVER charging (overvoltage) because the VR sees lower voltage and ramps up the alternator output

You could do this with a reversible modification. Unplug the blue wire at the ballast. Use that connector to trigger the relay. Wire the relay load output back to the ballast terminal. Also pull the VR connector off, and wire the relay output over to that. You can do this with push on "flag" terminals and not cut anything.

Feed the relay power from the "big stud" on the starter relay through a fuse/ breaker.