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HomeTips & GuidesTaillight Harness – How to Build and Connect It

Taillight Harness – How to Build and Connect It

The Taillight Harness is vital to your car’s lighting system because it shows your presence on the road and prevents accidents. 

However, you must have a working taillight harness for these lights to function properly and securely. 

Here, I will teach you how to build a rear tail light wire harness and connect it. 

How to Build a Rear Tail Light Wire harness

To start this discussion, let’s first build a tail light harness in case yours got destroyed.

Gathering materials and tools

First, buy multicolored 16 AWG wires to ensure easy connection and troubleshooting. 

Then, check your car tail harness diagram or the old harness to identify the necessary components, such as braking lights, left and right turning signals, grounding, and running lights.

 It would be best if you then gathered tools like wire strippers and cutters, connectors or a soldering iron, and heat shrink tubings.

Preparing the wires

Once you gather all the necessary tools and materials, you must prepare the wires. 

Check the wiring diagram to know which color codes you should use for each wire, including the ground.

 Now, determine the lengths of each wire from the vehicle’s main harness to the back of your car and cut accordingly.

 From there, strip about ¼ inch of the end of each wire for connection.

 A few electrical installation materials

 A few electrical installation materials

Connecting wires

Connect each light at a time to the corresponding wire on your vehicle’s main wiring harness. Once done, solder the connections and utilize the heat shrink tubing to ensure safety. Now connect the ground wire where all other car lights are.

Test, secure, and Install

You need to test for circuit continuity using a multimeter from the connection steps. Heat the shrink tubings to secure and insulate the connections if all is in place. Use several cable ties and bundle your wires together to form a harness. You may now use your car wiring diaphragm to connect and route the cable in your vehicle. Finally, test and see if each light works as intended after connection.

How to Connect the Tail Light Wire

In part, I will teach you how to connect a tail light wire to your towing trailer.

Preparation and material gathering

In preparation, you must first buy a 5- to 4-wire box converter that suits your trailer. The device costs about $20 and converts a 5-wire signal from your car to a 4-wire signal for your trailer. 

To be precise, it combines the braking and turning power into one wire without shortcircuiting. Then, you need to gather the right materials and tools for a successful endeavor: some of the tools you may need include:

  • Soldering iron
  • Utility knife
  • Heat shrink tubings
  • Snips
  • Adjustable wrench ‘
  • Flat head screwdriver
  • Wire striper

Access the tail light area

Before beginning the wiring process, place your car on a flat surface and engage the parking brake for safety.

 Then, disconnect the battery to prevent electrocution or short circuits. Next, go to the lining of the tail lights and locate the clips, which you can pop off. 

While peeling this lining, focus on the corners and edges to ensure you have a large working area.

Identify your vehicle’s tail light wiring.

First, note that each vehicle’s tail light wiring may be different. Therefore, you need to know what yours looks like and identify the grounding, braking, turning left and right, and running lights. 

You will need to use a multimeter to test your car model’s wires or wiring diagram.

Connecting the Wires

In this section, you will connect tail light wires to their correspondent on the converter switch. But before that, you will need to prepare them properly.

 To start, snip the ends of the tail light wires and that of the converter, then strip the ends carefully. Insert two heating shrink tubes (a tight and a bigger one) into the wires before you can join them. Once that is over, join the converter wires with the corresponding ones on the tail light and twist them together. 

Then, solder the joints carefully and place heat shrink tubings, starting with the tight one.

 Lastly, heat the tubings to secure the connection and attach the ground wire to a screw of the car body.

Testing and using

Now that the connection is successful, you must put everything where it was, including the battery connectors. From there, test your brake lights, signals, and the running light, each at a time. If all works as you want, you can legally use your trailer with your car. 

Using a multimeter to test a current

Using a multimeter to test a current

How to Fix Tail Light Wiring

While you may have properly connected your car tail light, they can sometimes have performance challenges. Luckily, I have researched and found steps to help you fix the writing successfully. 

Step 1

First, by switching on your parking lights, you must ensure only the tail lights have an issue. I recommend checking the fuse if you find that even the plate and parking lights are not functional.

Step 2

Once you are sure only the tail light is not working, it’s time to test it properly. First, remove its protective lens and replace the bulb with a new one. If nothing happens, then the bulb is not the problem.

 At this point, remove the bulb and use a test light or a multimeter to ensure the socket is not the culprit. 

If the tester or the multimeter shows some power indication, check if your grounding is correct. However, if nothing shows on the power testing result, follow the wiring to know where it has an issue.

Step 3

From there, disconnect the wiring harness of the first tail light and test if your car has power. If the vehicle power testing results are positive, the tail light harness has a problem.

 I recommend cleaning the connector and the terminal and then connecting again to check the harness inch by inch.

Connecting tail light connectors

Connecting tail light connectors

Step 4

When you’re sure this harness has a problem, separate the wiring on both tail lights and inspect each. 

Once you find the one with a problem, I recommend replacing it with a new one. Alternatively, connect a 16-gauge wire from the joint to the tail light contacts. 

While here, ensure the connection is snug and insulated to prevent short circuits and corrosion from a loose connection.


With this guide, you know how to build a tail light harness, connect it, and troubleshoot it when necessary. Remember always to observe electrical safety rules to prevent costly electrical hazards.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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