A yellow wire would not be original which suggests there may be more jerry rigged wires in your vehicle. So there is no telling where it went. I would put a test light on the wire and see if it lights up when you turn the ignition key to the right. If so, it probably went to the negative post of the coil if you still have the original 6 volt positive ground. If it doesn't have power, check to see if you have power to the ignition switch. If not, this could have been the power supply which comes from the ammeter. I would determine if it was the power source or wire to the coil and just run a new 14 gauge wire.
