Volts, Amps, And Ohms
In order to fully understand how the wiring in your car works, let's take a look at the element we're trying to harness-electricity. The three most basic units in electricity are voltage, current, and resistance. Voltage is measured in volts, current is measured in amps, and resistance is measured in ohms. A simple analogy to help understand these terms are plumbing pipes. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size.
Getting a little more specific, voltage is the force that pushes electrons-or units of electricity-through a wire. It is a function of the electrical source, which, in our cars, is a battery. While some late-'40s and early '50s cars may still have an original six-volt system in place (sometimes even a positive-ground arrangement), it's safe to say that if you're rewiring your custom project, it's likely getting a modern 12-volt, negative-ground system.
Current is the volume of electrons moving through a wire or circuit. Measured in amps, it will be dependent on voltage and resistance. The resistance in any given circuit is a physical characteristic of the electrical components involved. For example, the size and material of a light bulb's filament will determine how much resistance is in the bulb. All components will have a certain amount of resistance, although the resistance in wires, switches, and fuses is typically very minimal.
When discussing resistance, the natural segue is to define conductors and insulators. Basically a conductor is something that current can pass through; an insulator is something it can't. Consider these scenarios: If you ground a stereo to a plastic dash, it won't work because the plastic is an insulator; ground it to a clean metal surface and it will work. Note that we say clean metal because paint can be an insulator.
Not to confuse the issue, but there are situations when something can be either an insulator or a conductor. Your body is an example. If you touch both terminals of an automobile battery, nothing happens because you have sufficient resistance that 12-volts can't overcome. But if you've ever grabbed the spark plug wire on a running engine, it didn't take long to discover than 20,000 volts was more than enough.
 Is space at a premium under...  Is space at a premium under your dash? Compact fuse panels, like this one from Wire 1 Hot Rods, use OEM-style mini fuses and are designed to fit where standard fuse blocks won't. Several companies also offer harnesses designed to mount the fuse panel in the truck or in other out-of-the-way places. |
 Some wiring harnesses use...  Some wiring harnesses use bulkhead fittings to pass through the firewall; others are designed to use a simple hole and rubber grommet.... |
 ...You'll want to think about...  ...You'll want to think about which style you prefer when selecting a wiring kit. |
 It can be intimidating to...  It can be intimidating to look at a wiring harness as a whole, but focus closer and you'll notice that most kits label wires every few inches, making it easy to keep track of what goes where. |