How to Wire Your Car for Trailer Lights

Properly wiring a trailer improves the safety of a vehicle's passengers, other motorists, and the cargo in the trailer. Running lights, turn signals, and brake lights on a trailer are also required by law, so be sure they are set up and functioning properly before embarking on the next trip.

How to Wire Your Car for Trailer Lights

Learn how to locate a vehicle's wiring harness, and choose the appropriate connectors and adapters to set up any vehicle for proper towing. Standard, 4-pin connectors can handle lawful essential lighting, while additional pins allow operation of a trailer's brakes, reverse lights, and other operations.

Trailers and hitches are available from RV dealerships and rental agencies that specialize in travel equipment. Trailer kits, connectors, and other wiring accessories can be obtained from auto part stores and online retailers. Marketplaces like eBay offer trailers and accessories from both private and commercial sellers, and buyers can find a selection of used and new products. Learn to properly wire a vehicle for towing a trailer to ensure safety and compliance with the law.

Locating the Wiring Harness

A wiring harness is needed to connect trailer lights to a vehicle. This is built into nearly all modern vehicles and is located near the rear door or trunk. In some cases, the harness may be found around rear tail lights or a vehicle's hitch, if it has one. If the harness cannot be easily located, consult the vehicle's owner's manual. It should have specific directions for finding and retrieving the unit.

The harness is designed to be stored away when not in use and pulled out when needed. If it is located within the vehicle, it is specially coated with a material that allows it to be closed in a trunk or hatch door without it being pinched or severed. Once the harness is located, pull it out of its storage location and check to see if it reaches the connector on the trailer. Depending on the vehicle and the design of the trailer, it may be necessary to purchase trailer wire extensions.

Connecting the Harness

Most harnesses, by themselves, cannot be directly attached to a trailer. Instead, they are designed to be fitted with connectors. There are several ways to do this, though most users need to begin with either a T-One connector or a custom hardware kit

T-One Connectors

T-One connectors are the easiest way to establish a link between a vehicle's wiring harness and the trailer. These units are designed for specific vehicle makes, models, and years, and nearly all newer automobiles have a T-One connector available for them. T-Ones are designed with a 4-pin trailer connector, though they can also be outfitted with adapters that allow them to hook to 5-, 6-, and 7-pin connection types, as well.

Custom Hardware Kits

Vehicles that do not have a T-One connector available for them can still be hooked up to a trailer with a custom hardware kit. These can be built by custom vehicle shops and may also be found from online sellers. It is possible to build a custom trailer kit, as well, and directions for creating these units can be found online.

Owners who decide to design their own custom kit should be clear on whether a vehicle sends separate signals for turn and brake lights or not. If they are separate, a converter is needed to reduce the signal to the two-wire system used for controlling these functions on a trailer. Also, some vehicles do not generate enough power on their own to handle the trailer lights. In this case, look for a powered converter, or ModuLite connector.

Other Connectors

Although standard connectors like T-Ones are designed for a 4-pin connection, there are actually several types of connectors available. It is necessary to use a compatible connector for the trailer to properly use additional features like brakes or a hot lead. Connectors can have anywhere from four to seven pins, depending on the functions they provide.

Most common connectors are designed as either flat or round units. Flat connectors consist of a narrow component with a row of pins. Round connectors have their pins evenly distributed within the cylindrical shape of the connector. Learn about the different pin numbers and consider the trailer being used to choose the appropriate connector type.

4-Pin Connectors

4-pin connectors are capable of handling the essential functions of trailer electronics, and each pin handles its own task. One is responsible for the running lights, another is dedicated to the turn signals, and a third manages brake lights. The final, fourth pin is a ground wire. Four-way connectors are the standard for lightweight trailers and are used on all T-One connectors. They can be either flat or round, though flat connectors are more common.

5-Pin Connectors

5-pin connectors consist of the same pins used for 4-ways but include a fifth pin that can be used for other functions. This pin is normally reserved for handling surge or hydraulic brakes on a trailer. The fifth wire in a 5-way connector responds to a vehicle's backup lights and can disable the trailer's brakes when the vehicle is in reverse. As with 4-way connectors, 5-ways can be either flat or round.

6-Pin Connectors

6-pin connectors add two additional pins to 4-way units. These are normally used for control of trailer brakes and a 12-volt hot lead. 6-way connectors can be designed as round or square units. Rounds are more common on horse trailers, while squares are seen more often on campers.

7-Pin Connectors

7-pin connectors offer three extra connections, allowing users to handle electric brakes, a hot lead, and backup lights for their trailer. These connectors are only available in the round design, though they may have either round pins or flat pins. Of the two, flat pins are much more common and are standard on most trucks and SUVs built with a trailer hitch.


Many vehicles and wire kits are designed with a 4-pin connector. However, owners using a T-One connector can find adapters that can easily hook up to 5-, 6-, or 7-pin connectors.

Other Considerations

Connectors are sold with a diagram that indicates which pins perform each function. Diagrams can also be found online to help owners be sure they are connecting their harnesses properly. In some cases, the trailer connector may need to be mounted beneath the vehicle. In this case, look for a mounting bracket to protect the connector and keep it from hanging out beneath the vehicle and scraping on the ground.

How to Buy Trailer Kits and Connectors

Trailers, trailer kits, connectors, and other accessories for wiring lights can be purchased from sellers on eBay. Look for these items by entering keywords into the Search field on the homepage. For example, to find a flat, 4-pin connector, type "trailer connector 4 flat" into the Search field. Listings can be refined further by adding new keywords or by applying any of the category filters that appear on the search results page.

Examine item listings before committing to a purchase. In addition to the price of the item, be clear on shipping charges, shipping methods, and the seller's accepted payment methods. If you are buying a used item, take the time to inspect available photos and read the seller's description of the item. Contact the seller if you have any questions about a listing.

Review the histories of eBay sellers to find the most reliable users. Positive feedback ratings and number of completed transactions are displayed in item listings, though it is also possible open sellers' full profiles by clicking on their names. Full profiles display comments left by previous buyers, as well as average ratings they have given the seller based on several aspects of the user's performance. Once you have received your item, leave feedback about your experience. This helps the seller improve and helps future buyers make an informed decision.


Properly wiring trailer lights for a vehicle promotes the safety of passengers and other motorists. Brake lights, turn signals, and running lights on a trailer are also required by law. Trailer lights can be can connected to almost any vehicle. Begin by locating the wiring harness in the rear part of a vehicle. This may be in the trunk of a car or the rear area of a hatchback or SUV. In some cases, the harness may be near the tail lights or a hitch, if the vehicle has one. Once the wiring harness is found, shop for a T-One connector that fits the make and model of the vehicle. If none are available, consider a custom kit. Choose the appropriate number of pins for the type of towing that will be done, and purchase wire extensions and adapters, if needed.

Trailer kits, connectors, adapters, and other components can be obtained from auto repair shops, rental companies that specialize in travel equipment, and online sellers like those on eBay. Shop around to find the best prices and products, and research compatibility with a vehicle and trailer before purchasing any custom kits. Proper installation of trailer kits and connectors helps ensure safe travelling and transportation for the next towing trip.