Wiring a vehicle can be pretty intimidating. Just getting the wires where they need to go can cause a serious case of agita. The harness manufacturer will bundle the wires so most of them go where they are supposed to, but there are always a few circuits that need to go somewhere else. You also need to protect your harness from hazards like sharp edges, screws, and pinch points.

Having wired more than a few cars in my day, I’ve collected some tips and tricks that will help make your harness install go a lot smoother.


As mentioned before, the layout of your harness is usually determined by the manufacturer. If you’re installing a factory replacement or application-specific harness, you should be good to go. But if you start adding, deleting, or moving circuits, you’ll need to do some wire rearranging. If you are using a universal-style wiring harness, you’re pretty much on your own when it comes to routing.

Regardless of what type of harness you have, the first order of business is to lay it out on the floor and see how the circuits are bundled. This allows you to determine if you need to rearrange or add wires to the harness to suit your vehicle. Don’t forget to label each wire (if not already labeled) and bundle. It is really easy to forget what section you’re working on and mix up circuits.

Jefferson Bryant Wiring Harness Routing Story Harness on Floor
It looks like multi-colored spaghetti, but a vehicle wiring harness isn’t that hard to understand. The best way to do that is by laying the harness out on the shop floor and breaking it down into bundles of circuits. Most aftermarket and reproduction wiring harnesses come prebundled to save you time and effort, but it’s still a good idea to lay it out and familiarize yourself with the routing, wiring color coding, and connectors. (Image/Jefferson Bryant)

You can use zip ties to secure bundles as you go, but don’t permanently cover them in tape or wire wrap. If you do, Murphy’s Law will bite you hard and you’ll end up pulling it all apart to move one wayward wire.

Routing and Protecting the Harness

Once you have the harness bundled to suit, you have to get it in the car. The easiest method is to lay the harness on the floor in the passenger area. That way, you can determine where to the main harness and fuse block will go and get an idea where the individual bundles will go.  

Jefferson Bryant Wiring Harness Routing Story Harness in Car
Once the harness is in the car, it is a good idea to break the big bundle into smaller location-specific bundles. Coiling the wires together makes it a lot easier to work without getting tangled up in 4,000 feet of wire. (Image/Jefferson Bryant)
Jefferson Bryant Wiring Harness Routing Story Labelled Wires
If the wires/circuits are not already labeled, make sure to do so. That way you won’t have to track everything down once the harness is in the car. This is doubly important if you aren’t going to use a circuit immediately but want to save it for future use. (Image/Jefferson Bryant)

Once the main harness is in position, you can start routing the smaller bundles. Any time you pass wiring through a metal hole in the firewall or other sheetmetal, you must use grommets to protect the wire from cuts and abrasion. Summit Racing has a wide variety of grommets and seals for this purpose.

Be aware of other potential hazards when routing the harness. Headers and exhaust systems are really good at melting wire, so keep your wiring at least six inches away from any major heat source. If you have to route wires closer to hot spots, opt for some heatproof shielding to protect them.

Jefferson Bryant Wiring Harness Routing Story Firewall Grommet and Wrap
Sometimes you have to create a large hole to route a harness. We plan to swap in a GM Gen. V LT1 in this Chevy OBS pickup and decided to route the harness though the factory clutch rod location in the firewall. We enlarged the hole with a hole saw, cleaned up the burrs, and installed a grommet to protect the harness from cuts. Wire loom will not provide enough protection—always use grommets wherever wires pass through sheetmetal! (Image/Jefferson Bryant)
Jefferson Bryant Wiring Harness Routing Story LS Engine Harness
This LS engine harness has several sections that need to route to different areas. Each section was taped every few inches to keep the wires together. (Image/Jefferson Bryant)

Moving parts like linkages, steering columns, shifters, and fans can really do a number on wires. Make sure wiring is secured well away from moving components. You would be surprised how easily something like a steering column rag joint can snag a wire and yank it right out.

Jefferson Bryant Wiring Harness Routing Story Wrapped Wire Under Car
Make sure to keep wiring under the car away from hot and moving parts. Give yourself at least six inches of clearance from the exhaust, driveshaft/axle, steering, and suspension components. Watch for the weld pinch points too. (Image/Jefferson Bryant)

Universal chassis harnesses often have circuits you don’t need or want immediately. You could eliminate those circuits by removing the wiring entirely in the layout stage, but you will likely regret that later on. It is a good idea to make a small bundle of such circuits and tie them up under the dash or the carpet. This way they’re ready to go if you decide you need them.


Once you’ve routed all of the bundles, you can start wrapping the harness. Summit Racing offers a ton of options including wrap and sleeving and plastic and cloth tape. I prefer fabric harness wrap. It’s easy to use and allows you to access the wiring—just pull it off, do what needs to be done, and slip it back on. It has some spring to it, so it holds itself closed while being incredibly flexible. I really like Summit Racing’s Woven Fabric Wire Wrap Kit. It comes with wrap in several diameters, zip ties to secure bundles to the vehicle, plus electrical and self-vulcanizing sealing tape to seal the ends and joints if necessary.

Jefferson Bryant Wiring Harness Routing Story Engine Harness Wrapped
Flexible wire loom is easy to work with and protects the wire from abrasion. It is split so you can access the wires, but springs back and holds itself closed. We used self-vulcanizing tape to wrap the main and individual branches. After a few days, the tape bonds with itself so it won’t unravel. (Image/Jefferson Bryant)
SUM-890357 Summit Racing Wire Wrap Kit
Summit Racing’s Woven Fabric Wire Wrap Kit is ideal for wrapping a large harness. It comes with 1/8-, 1/4-, 1/2-, 3/4-, and 1-inch diameter wrap in various lengths; four- and eight-inch long zip ties to secure bundles to the vehicle; and electrical and self-vulcanizing sealing tape. (Image/Summit Racing)

Other Considerations

Keep future access in mind when you route your harness through the car. Front to rear runs should be located at the interior edges of the rocker panels. Nobody likes a rats nest or wire hanging from under the dash, so take the time to secure the dash loom where it will be accessible and not covered by bracketry and components.

Jefferson Bryant Wiring Harness Routing Story ECU Wiring
Think about access when routing the wiring harness and mounting electrical/electronic components. Keeping things clean and neat will make tracing wires or servicing components much easier, and it looks good too. This ECM panel mounted under the seat of a squarebody Chevy truck is a great example.(Image/Jefferson Bryant)
Jefferson Bryant Wiring Harness Routing Story Wire Under Dynamat
Sound deadener is a great way to help secure the harness without using screw-on clamps, especially well on floors and trunks. Braided flat sleeve wire wrap is perfect to protect wiring without creating lumps underneath the carpet like round wrap can.(Image/Jefferson Bryant)

Ground terminal strips and quick-disconnect plugs are great ways to add serviceability to a harness. Instead of tracing a wire, you can simply unplug a section to eliminate potential fault points. Adding quick-connect plugs for the starter, fuel pump, door wiring, and even the entire engine harness makes servicing these items a snap. You can even add fused terminal strips for 12-volt positive constant and hot feeds for accessories.

Jefferson Bryant Wiring Harness Routing Story Ground Terminal Strip
Ground terminal strips like this are great for eliminating ground loops and a zillion ground screws all over the car. They have a cover to protect the strip from dead shorts. You can get terminal strips for 12-volt positive connections as well. (Image/Jefferson Bryant)
Jefferson Bryant Wiring Harness Routing Story Molex Plug
Adding quick-disconnect plugs like this 12-pin Molex plug makes servicing components like electric fuel pumps, gauges, lights, and even engine harnesses literally a snap. (Image/Jefferson Bryant)

Routing a wiring harness isn’t difficult if you take your time and stay focused on the job at hand. When you’re finished, you’ll have vehicle wiring that has solid connections, routed neatly, and easy to access if issues arise.

Jefferson Bryant has been a full-time automotive journalist since 2003. He has written countless how-to articles, nine books, and built several award-winning vehicles.