Over time, many things can cause electrical problems with your Mustang. Components get old and worn, or a wire can rub against something and expose itself to metal. I've seen cases where body shops inadvertently crimped a wire while installing a body panel. Sometimes during mechanical repairs, wires can get in the way so they're haphazardly shoved aside. This is unfortunate because wiring is a car's nervous system. If it isn't in good shape, then things won't work properly. In the worst-case scenario, it can cause an electrical fire.
A self-powered test light is handy for locating shorts with the battery disconnected.
One common condition that can cause problems is an electrical short. Let's examine what a short is and how to find one.
Electrical current normally flows from the positive battery cable to the starter solenoid, through the main wiring harness into the dash, through a fuse, then to an electrical device, and finally to ground, which is either the metal body or a ground wire in the wiring harness that attaches to the body. In an electrical short, current flows from the battery directly to the body chassis or metal, which is a direct ground and full unchecked amperage. In that case, it has taken a shortcut to ground, thus creating an electrical short circuit.
A wiring diagram is helpful for tracing shorts. Virginia Classic Mustang now offers Colori
If a wire is shorting to ground without going through a fuse, it will get extremely hot and burn through if current flows long enough, which is sometimes only a few seconds. If a short exists on a fused circuit, then the fuse gets too hot and quickly melts the element inside, thus breaking the circuit and protecting its wires. When a fuse blows for any reason, there's obviously a problem. Be happy that the fuse is doing its job and giving up its life for the cause, otherwise wires can melt, or worse: an electrical fire can occur.
Whenever a circuit is blowing a fuse, don't wrap the fuse in foil or a metal gum wrapper to keep current flowing through that circuit. This can cause the wire to get too hot and possibly take some other wire with it when it melts, causing more problems than you originally started with. If a fuse is blowing, the cause needs to be found and corrected.
To locate a short, the best tools are a wiring schematic for your Mustang and a self-powered test light, or continuity tester. A self-powered test light is similar to a test light to check voltages but is used with battery power disconnected. It has its own small battery and light, which illuminates when it detects a wire that's touching ground somewhere. It has a sharp probe on one end to pierce wire insulation if necessary; an alligator clip on a wire is at the other end. The alligator clip attaches to metal, such as the body or dash, to ground the test light.
Here is the method I prefer because battery power is disconnected. In Test Scenario A, after you disconnect both battery cables at the battery, remove the fuse that's blowing. Using the wiring schematics, determine the components that are on the circuit the fuse protects. Each one will have a color-coded wire for power and usually a black wire for ground. Unplug the connectors from each component. If one of the components is a light bulb, remove the bulb from the socket.