Trailer Brake Control Wire Harness

What the harness does

An electric trailer brake controller sits inside the cab and sends a signal to the trailer's brakes in proportion to how hard you brake. The controller has to connect to the vehicle's brake light signal, battery power, ground and the trailer connector output. A wiring harness makes that connection by plugging into a socket the vehicle manufacturer provides, instead of splicing wires under the dash.

How to choose

Match the harness to the vehicle, and then to the controller. Vehicle-specific harnesses are made for particular brands and model years, such as Chevrolet, GMC or Ram trucks with a factory tow package. Controller-specific harnesses connect a particular controller series, such as the Tow-Pro line, to the vehicle's connector. Universal or adapter harnesses help connect other controllers to a vehicle plug.

• Vehicle-specific plug-and-play: Plugs into a tow package connector behind the dash, with no cutting.
• Adapter harnesses: Convert one connector style to another so a controller will fit.
• Tail light and 4-way converters: A separate item for trailer lighting, and not the same as a brake control harness.

Specs to compare

Look at the stated vehicle fitment, the connector type, and the harness length. Check whether the harness is for a factory tow package, since vehicles without a tow package may not have the matching connector. Confirm the compatible controller series, and note compliance statements where listed. Make sure the harness handles the current of your trailer's brakes, which depends on the number of axles.

Installation notes

Most plug-and-play installs involve locating the connector behind the dash, plugging in the harness, mounting the controller within reach of the driver, and setting the gain. Do not mount the controller where it can interfere with airbags or pedals. Disconnect the battery before working on wiring, and secure the harness away from moving pedal linkage.

After installation, adjust the gain with the trailer connected, following the controller manufacturer's procedure, and test in a safe area at low speed. Trailer brakes are a safety system, so have a professional check your work if you are unsure. Confirm fitment for your exact year, make, model and trim, and verify the connector on your vehicle matches the harness. Check local regulations about trailer brake requirements, which vary by state and trailer weight.

Testing and common mistakes

After the harness is in place, test before towing. Many controllers have a manual slider or button that applies the trailer brakes without the vehicle brakes, and this is a good way to confirm the connection. Connect the trailer, apply the manual control, and listen for the magnets in the trailer brakes to engage, or use a test light or meter if you are comfortable with that.

Common mistakes include selecting a harness for the wrong model year, since connector styles change, and assuming a harness fits any controller. Another is routing the wires near the pedal or steering column, where they can be pinched or snag. Leaving the connector unseated or partially engaged can cause intermittent faults that are hard to trace, so listen for the click and tug gently to check.

Consider how many axles your trailer has, since the controller and wiring should be sized for the load. Check the trailer plug and wiring for corrosion regularly, and keep the vehicle side connector clean. If the controller displays an error, check the harness first, then fuses and grounds.