The 66 Mustang engine gauge harness connector, the big one that plugs into the main underdash harness through a big rectangular hole in the firewall, Has 8 bullet connectors in it. 6 of them are male and two of them are female. The two female connectors are numbers 4 and 8 in the drawing. Here is where all of those wires go. And yes, there are 9 wires coming out of those 8 connectors. The one with two wires coming out of it, #3, is made that way so that you can actually start your car. When the key is in the 'On' position, power goes through that red/green wire from the ignition switch out to the coil. However, when the key is in the start position, there isn't anything coming from the switch to coil, so, when you turn the key to start position, the coil isn't getting any power, which means that your car will just crank and crank, but never start. When the key is in the start position, power goes from the ignition switch to the starter solenoid along the red/blue wire, which engages the starter solenoid. That makes two things happen. One, power goes straight from the battery to the starter through the main circuit of the solenoid, and, two, power leaves the solenoid along that brown wire that's plugged into the 'I' post on the front of the solenoid. Inside the firewall connector, that brown wire is spliced into the red/green wire that goes to the coil, so power goes from the solenoid to the firewall connector, then out to the coil along that red/green wire. This also a full 12V charge, instead of the 6 or 7V charge that comes to the coil from the ignition switch because of the resistor wire. The resistor wire is there to extend the life of the breaker points and the coil. If the coil was getting the full 12V all of the time, those components would fail a lot sooner than they have to. The full 12V while starting makes the car easier to to start.
