Relays
A relay is a component that allows low current to activate a much higher current circuit without overloading a switch. They are commonly found in circuits for headlights, air conditioning, power windows, electric fans, or other components that require a lot of amperage to start and/or run. Another way to describe a relay is as a "switched switch" or "triggered switch" capable of handling higher amperage than the switch it's replacing. A typical relay only needs about 120 milliamps (0.120 amps) to be triggered and held closed, but can supply 30, 40, or more amps (depending upon its rating) to the load being fed. By making the relay the main switch in the headlight circuit and using the headlight switch as the trigger for the relay, we shift the amperage load off the headlight switch and onto the relay, which is designed to do the job. It's basically cheap insurance that increases switch life and prevents circuit damage. You generally only want to run one component from a given relay, but you can use a single switch to activate multiple relays.

Selecting A Harness
Now that we've laid out the electric fundamentals and functions of primary components, let's help you use that knowledge to select the best harness for your custom project.

We're going to proceed under the assumption that you want more than just a stock replacement harness for your custom. Besides typically costing more than universal harnesses, OEM-style harnesses tend to use outdated technology and lack the circuits and capacity necessary for the accessories we add on. In some instances (Tri-Five Chevys, for example) you can get specific-fit aftermarket harnesses that fit in original locations and feature additional circuits, spade fuses, and other modern elements. Universal-style harnesses that can be tailored to the needs of the project car at hand, however, will better serve most modified '50s and '60s vehicles.

One of the first things you'll need to consider is how many circuits you'll need. Twelve circuits seem to be a standard baseline for aftermarket harnesses, but it's common to see kits with 15, 18, and even 20 or more circuits. How many circuits do you need? Twelve are plenty for basic custom and rod projects without a ton of accessories, but you may want to step up to 18 or more if your car is going to have power windows and locks, electric seats, or other creature comforts. It's generally easier to work with too many circuits than too few; extras can always be tied off for possible use in the future, or cut out completely.

Another key consideration has to do with harness construction and fuse block configuration. Some aftermarket harnesses come with all connections pre-terminated to the fuse block; others come with the various wiring harness bundles loose, leaving the installer to route them to the fuse block as he sees fit. While some may argue that the latter style is more labor intensive, others feel it offers more flexibility and allows for a cleaner installation. It's largely a matter of preference, but it's something to consider as you're shopping around and exploring your options.

Aftermarket fuse panels come in varying sizes and shapes, too. And while space is usually not as tight in '50s and '60s cars as it may be in earlier vehicles, you'll want to think about your mounting location and take some measurements before making a decision. The nice thing about most standard-sized fuse blocks is that they have provisions for mounting necessary flashers and relays. Compact fuse panels often require you to find remote mounting locations for such items. Nothing says your fuse panel must be under the dash, either. Several companies offer harnesses with extra-long wiring that allows the fuse panel to be located under the package tray or in the trunk.