Time for reading: 5 min.

Connect Your Car Lights To Your Trailer Lights The Easy Way

We discuss wiring harnesses & adapters that mate tow vehicle and trailer tail lights so they work in sync – even if your truck’s not pre-wired, or connectors don’t match.

When it comes to "electricity", many people are either scared silly of it, or run the other way rather than try to learn about it. Since it is a powerful force, it certainly is something to be respected. For us vehicle owners, perhaps we understand that our cars and trucks have a battery under the hood that needs occasional replacement, and light bulbs that may burn out after several years. Beyond that, many of us are ready to leave any electrical work to the "experts".

Trailer Hitch Wirings Banner

If you have recently purchased a trailer, "electricity" will eventually become a question you'll need to answer, as in, how do I connect my trailer's lights to my tow vehicle's lights? It may seem obvious that your trailer has tail, turn, and brake lights at the rear which need to operate in sync with your car's lights. That isn't going to happen by magic. We are here to share some wonderful news with you: for the vast majority of vehicles on the road, CARiD has made it quite easy to 'make the connection'. Follow along and discover how simple it is to connect these two systems together. We will be looking at specific components within our Trailer Hitch Wiring & Electrical Store.

A typical trailer harness plug is a "4-flat" plug with 3 male terminals and 1 female terminal.
In order to mate with a trailer's 4-flat plug, your tow vehicle also needs a 4-flat plug, in this case, one that has 3 female terminals and 1 male terminal.

The scope of this article will presume that your trailer has what the industry calls a "4-flat" wiring connector, which is the standard on many new trailers sold in the U.S. This article will explain the purchase of the correct harness for your vehicle, so that the two can be joined. The trailer plug should be a '3-male, 1-female', and the tow vehicle plug should be the opposite, or '3-female, 1-male'.

Determine What You Have

Factory Trailer Harness Connector
An unused factory trailer harness connector is shown here on this pickup truck, located near the spare tire.

A good first step will be for you to understand if your car or truck is equipped with factory trailer wiring. If you purchased it new, this should be fairly easy. Many pickup trucks and SUVs which are suitable for towing are available with factory trailer packages. In fact, the Big 3 domestics (GM, Ford, and Chrysler) have made this harness standard equipment on many of their latest full-size pickup trucks. Look at your Monroney label which will list any optional equipment. If you purchased your vehicle used, or are simply unsure of what you've got, you can contact your vehicle manufacturer's Customer Service Department. You can also look under the rear bumper and/or in the rear storage area for an 'unused' plug.

To further help the consumer and provide uniformity and consistency, the Big 3 formed USCAR (United States Council for Automotive Research) in 1992, to work on areas of technology of mutual interest. USCAR created a design for a uniform trailer connector which has been in use since the late 1990s. If your vehicle has an USCAR-style trailer connector on it, the selection and purchase of an appropriate harness just became easier.

Uscar Trailer Harness Plug
The USCAR trailer harness plug, standardized among GM, Ford, and Chrysler Corporation vehicles.

Let's now look at some examples of vehicles, and the possible scenarios which can exist. We'll pick one vehicle from each of the Big 3 (GM, Ford, and Chrysler), and we'll look at different levels of standard and optional equipment, as well as availability of splice-free harness solutions.

Scenario #1: The Vehicle Comes Standard With Factory Trailer Package

Dodge Ram Trailer Harness Connection
The heavy-duty Ram pickup trucks have a trailer harness connection in the rear bumper from the factory.

If you have purchased a new Ram 2500 pickup truck, congratulations! You have selected one of the most stylish and capable full-size pickups available today. Since you are planning on doing a fair amount of towing, your decision is doubly-smart, as all current Ram 2500 and 3500 trucks have a standard-equipment trailer harness connector in the rear bumper.

Tow Ready Trailer Wire Harness
This Tow-Ready trailer wire harness has an USCAR connection on one end, and a 4-flat trailer connector on the other. It doesn't get any simpler than this.

For this truck, all you need is a harness with the appropriate USCAR connector on one end, and a 4-flat connector to mate with your trailer on the other end. Tow Ready, a division of Cequent, has the exact harness to complete your connections. Estimated installation time from the manufacturer? Five minutes.

Scenario #2: The Vehicle Does Not Have Factory Trailer Package, & T-Shaped Harness Is Available

You recently scored a sweet deal on a 2013 Buick Enclave, "fully equipped" as per the salesperson at the dealership (who would only tell you the truth, right?). Soon after you got it, you and your significant other decided to get that boat you've been dreaming about, complete with trailer. Now you're faced with figuring out how to connect the boat trailer's wiring to the Enclave. You then discover that while Buick offered the option of a factory tow package, yours does not have it.

Buick Enclave 13

Because the popularity of towing has been combined with customers' reluctance to cut their truck's wiring, the trailer equipment manufacturers have come up with a creative solution: the so-called T-shaped wire harness, which is installed with no cutting or splicing of wires. The concept is brilliant in its simplicity: since the tow vehicle's tail lights have all the "connections" we need for the trailer (turn, brake, and daytime running lights), you unplug the tail lamp harnesses, plug in the T-connectors to both the vehicle harness and the tail lamps, then route the 4-flat harness to the rear. The Curt brand T-Connector (available in the Product Options field) gets this job done for your Enclave.

Curt T Connector Harness
A Curt brand T-connector harness. After unplugging the tail light connectors, the 2 connectors near the yellow wire are plugged into the harness and lamp on the driver's side. Same is done on the passenger side with the connectors near the green wire. The 4-flat plug on right is then routed out to the rear bumper.

While this is slightly more time-consuming because of the need to pull down some interior panels for tail lamp access, Curt estimates an install time of only 45 minutes. When you're done, the needed 4-flat connector is ready and waiting at your rear bumper.

Scenario #3: The Vehicle Does Not Have Factory Trailer Package, & T-Shaped Harness Is Not Available

If you are fortunate enough to live in the western part of the U.S., where body rust takes much longer to get its grip (and we in the east envy you!), you may still be driving something like a 1979 Ford F-series pickup truck. If you've had it for a while, you fully understand what a workhorse it is. And even at this late stage of its life, you may be considering the installation of a trailer hitch to get more work out of it! However, let's remember that the truck was built when Jimmy Carter was president. If you were alive then, that comment is guaranteed to make you feel old!

A "Factory tow package" didn’t exist for this ride. And since most trailer suppliers are making T-shaped wiring harnesses for later models, vehicles from 1979 need not apply. However, all is not lost. There are universal trailer wiring kits that are not only extremely affordable, they are easy to install. For our purposes, another beautiful thing about this truck is that it uses "combination" running (tail) and turn signal bulbs, as do most trailers, so the connections are that much easier.

Hopkins 4 Flat Connector Kit
The Hopkins 4-flat Connector Kit comes complete with harness, electrical connectors, and its own test light!

Hopkins Towing Solutions manufactures a number of 4-flat wire connectors that can get this job done. If we look at all the choices in the Product Options menu, the Connector Kit which includes wire splicers and cable ties is the best choice. It even comes with its own circuit tester! Just find the wires which are "hot" when that function is in use, crimp down on the wire splicers, and run the 4-flat connector out to your rear bumper. Although Hopkins does not publish an estimated time to complete the job, it should take just under an hour - and that includes pouring a 2nd cup of coffee.

Other Options

There are trailer wiring harnesses which have connector terminals other than 4-flat or USCAR. Your vehicle may have a 6-round or 7-round connector. Your trailer may have a 5-flat connector. The diagram below is not intended to be a wiring diagram per se, but rather is shown to give you an idea of what these choices look like.

Wiring Diagram

Should you need, for example, to convert your tow vehicle from a 4-flat to a 6-round, Curt makes such a conversion harness. Need a 4-flat-to-5-flat conversion? Hopkins has you covered with their adapter wire. Or how about a 5-flat to a 7-round RV style? Wesbar, a Cequent brand, makes this exact jumper connector for you.

Trailer Lights Conversion Harness

The point is that just about any two different trailer harness connectors can be joined by perusing the CARiD website. In order to purchase the correct harness, you simply need to examine the harness ends on your vehicle and trailer, and buy the one harness with the correct plugs on each end. Any questions, and an answer to your query is a phone call away! We hope this article has shown you that connecting the electricals between your tow vehicle and trailer need not be difficult to understand or to accomplish. CARiD is here to help you 'make the connection'.

Items Discussed in Article
Shop Now
Low Prices Price match guarantee
Guaranteed Fitment Always the correct part
In-House Experts We know our products
Easy Returns Quick & Hassle Free

All manufacturer names, symbols, and descriptions, used in our images and text are used solely for identification purposes only. It is neither inferred nor implied that any item sold by is a product authorized by or in any way connected with any vehicle manufacturers displayed on this page.