You are missing your starter switch. "A" is where the switch should make contact and it will be covered by the switch when it is installed. "B" and "C" are the screws that hold the switch in place. There will be a large terminal on the switch for the positive cable. (if this is a GMC make that negative cable)
Another wire(10 guage) will run from there to your amp guage. The wire from the coil should run to your ignition switch.
