Wiring diagram will show two ignition circuits from the key switch.
Ignition 1 is key in run postion. Power goes to a connector on the ballast resistor (usually a blue wire) then divides. Some goes through the ballast resistor to the coil, some to the voltage regulator (usually a blue or blue with white trace). So that connector on the ballast has two wires.
Ignition 2 gets power when key is in Start position. That's the brown wire. At the connector on the ballast resistor it joins with the wire to the coil.
The double ballast resistor above is something added. Those were used on factory electronic ignitions in the mid 70s. One resistor helps regulate current that runs the amplifier (ECU), the other regulates current to the coil. Newer '4pin' ECUs have internal regulation and don't need the second resistor. If the engine is running a points type distributor or magnetic pick up with a later ECU, then you only need to have one resistor. The resistance of the resisistor should be matched to the coil.
