Many questions seem to arise concerning this seemingly simple component, such as "Where is the main power feed?' or "How do I replace this with the more modern blade type fuses?" First, there is no 'main power feed'. Here is how the wiring is on the 65/66 Mustang fuse box.
In the bottom left corner of the picture there is a contact with a black wire with a yellow stripe attached to it. There is a black/yellow wire coming from the hot side of the starter solenoid, the same terminal that the positive battery cable is attached to, which comes through the firewall, splits into three wires, one black/yellow, one black/orange and one yellow. The black/yellow wire on the fuse box is the black/yellow wire coming from the starter solenoid. This is constant power, whether the key is on, off or in the accessory position. The power is always on, thus, the term, 'Constant Power'. (insert smiley face.) You will notice that this contact has two round thingies on it. If you flip the fuse box over, you will see that one of those round thingies is the hot side of the fuse for the cigarette lighter/emergency flasher circuit, and the other is the hot side of a circuit called 'Dome'. A 65/66 Mustang does not have a dome light. This is the interior courtesy lighting which comes on when you open either door. On the other end of the Dome fuse is a green/yellow wire. This green/yellow wire goes out to the driver's side door jamb switch. On the other end of the cigarette lighter circuit is a blue/white wire. This goes out to the lighter, and splits off before it gets to the lighter and also goes to the emergency flashers, if your car has the emergency flashers. Not all 65 model Mustangs had emergency flashers.
On the middle fuse you will have two blue/red wires. One of them brings power directly from the headlight switch and the other goes to the instrument cluster back-lights. Those are the ones that come on when the headlights come on, not an alternator/generator warning light, or the oil pressure warning light.
In the top right corner, there is a black/green wire. This brings power from the ignition switch when the key is in the on or accessory position. Again, the wire is attached to a contact plate with the two round thingies. The one on the top is the hot side of the accessory fuse. There should be a wire hanging out of the front of the fuse box on that circuit which supplies keyed power to whatever accessories are plugged into that wire on the other end. The other round thingy, marked by the brown/green arrow, is the hot side of the heater fuse. There is a brown wire coming out of the other end of this fuse. If you have three speed heater fan, this wire goes out to the fan motor. If you have a two-speed, center-position/off heater fan, this wire goes to the fan switch. The fuse box of a 64 1/2 is a little different in a couple of ways. I'll address that one in another post.
So, as you see, power goes to the fuse box from three different places, the starter solenoid, the ignition switch, and, the headlight switch, and the systems that get power from one are pretty much completely isolated from all of the others. There is a little more involved in replacing this with a blade type fuse box than just removing this fuse box and sticking the new one in. The new one needs to either have a provision for isolated independent power feeds for different circuits, or, it needs to be more than one fuse box.
