If you already know the reasons why you would want a custom headlight harness and you just want to get to the "how" part, here is a link to skip directly to the diagram and instructions on how to build it:  

Headlight Wiring Harness Project

Nearly all vintage car owners have experienced the issue or poor light quality with standard, factory installed sealed beam headlights.  While some may understand the causes, many do not realize that this problem can be easily fixed. For many classic car owners, the option to change out or upgrade original type components to improve headlight function is not even a consideration. But, the fact is that there are no changes necessary. The simple solution is to add a modular headlight relay wiring harness that will provide good clean power to your vehicles headlights, without any modification to your existing wiring. That’s right, no alterations to your valuable collector vehicle!

You Get What You Pay For

There are hundreds of companies online offering a “universal” harness, and most of them have an amazing low price. Let’s start by stating the obvious, you get what you pay for.  If the unbelievably low price did not scare you away, then the word “universal” should be the next thing that makes you turn tail and run. Anything built to fit “everything” usually fits nothing well. You can bet on the fact that the locations of the plugs and connections will not allow you to locate the harness in the right places under your hood. If you are “lucky”, you will have one or two significant lengths of "extra wire" that you have to coil up and try to hide somewhere under the hood. Why is that lucky? Because the alternative means that your “universal” harness was too short and required you to route it by heat sources, or moving parts, or worse yet, to drill a holes in the vehicle to get it to where it needs to go. 

Benefits of a Custom Harness

There are several reasons for building a custom, here are the most common:

• Custom harnesses allow the installer to route the wiring in a manner that is best for the specific vehicle. This means around things that can cause issues, like chafing, heat damage, or other elements. Done properly, a harness can be added with nearly no evidence that it exists. The wiring can be run and the relays and fuse can be placed in areas where they are nearly impossible to see.   For many vintage car owners, keeping the stock appearance makes this reason enough by itself.

• Custom harnesses allow the builder to use premium parts that provide increased reliability. Using high quality relays is on the top of this list. Those cheap harnesses typically use $2 imported relays that are prone to quick failure. When the relay dies, you have no lights. In addition, your harness should be properly fused. Using a quality made weather proof fuse holder is a must. In addition, the quality of the wiring makes a world of difference. Low quality imported wiring is often made from a very low grade of copper, or aluminum and as a result, it cannot carry the power that your system requires. This type of wire is normally susceptible to fatigue and failure from heat, moisture and even normal use! Using a quality made copper wire will ensure long and proper function of your harness.  

• Custom harnesses allow the builder to have control of the quality of assembly. When building a harness yourself, you can be certain that it is done right. Those inexpensive harnesses made in China, are often poorly assembled. Issues include incorrect wire crimps, poor soldering, non-insulated connectors and many other issues. When assembling your own harness you have the ability to inspect and test each connection as it is made.

• Finally, the ability to customize your harness to meet your budget. Some customers may have a different list of priorities. While some customers will want to use braided fire resistant wire loom and polyolefin heat shrink tubing to encapsulate the wiring, other customers may be satisfied with inexpensive corrugated plastic wire loom and electrical tape. Some customers may be installing high powered halogen headlights and prefer to use 12ga wiring and ceramic headlight plugs instead of the standard 14ga wiring with plastic plugs. With a custom harness, the builder can alter any aspect fit their project.

How do I build my own harness?

Here is a link to the page that will help you identify the parts needed to complete that project in the manner that best suits your needs:

Headlight Wiring Harness Project