Easily Tap Trailer Lights Into Your Tail Lights




Introduction: Easily Tap Trailer Lights Into Your Tail Lights

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The goal of this ible is to show you how to wire in a 4 wire trailer plug into your tail lights. Installing a hitch is fairly simple, and even if you have an accessory hitch (for bike racks etc.), the electrical harness is a big up-sell by some installers. Yet, you can do it in under an hour for under 25$.

Full video now included.

Step 1: How It Works

You will need to buy an inexpensive box that takes the 5 wire input, and changes it to the 4 wires the trailer plugs use (essentially it merges the turning and breaking signals on a single wire, while preventing a short on the car's since since it uses two different circuits there).

Step 2: Material

You will need

Step 3: Remove the Lining

There are clips around the lining, which you simply pop off. In my case I had two kinds, you will most likely have one of the two or both. You should focus on the corners, make sure you remove enough to be able to peel away enough of the lining to have enough room to work in.

You will also have to remove along the edge if you want to hide the one wire that crosses over to the right side (more on this in the wiring stage).

Step 4: Figure Out Your Wiring

The exact wiring will depend on your car model. You can do this with documentation, or by testing with a multimeter. I did both.

In my case I googled it and found a list of the wire that correspond to the break, turn (x2), ground, and running lights. To make sure everything added up, I connected the negative lead of my multimeter to the ground (the body of the car), and touched the pins to measure if the voltage changed when I was using the function (break, turn signal).

In case you have a yaris as well, here are the wires.

  • ground: the screw in the body of the car (as for all cars)
  • break light: the purple wire (back), from the front it would be the bottom right pin
  • left turn light: teal wire (back), from the front, bottom left pin
  • right turn light: on the other connector (right tail light), it is in the same place as the teal wire, but it was yellow
  • running lights: the green wire (back), from the front, that is the top left pin (not the empty one obviously, the one next to it)

Step 5: Splice the Wires

Once you know your wires, splice them. It goes as follows:

  1. snip the wire, strip both ends and the corresponding wire on the unit
  2. twist the wires to make your life easier
  3. slip tight fitting shrink tube on both of the ends of the wire you snipped, as well as a oversize one (about 1/4 inch) on one of them
  4. twist all three together, and bend one back so it's a line, not a bend
  5. solder
  6. push the shrink tube in place
  7. heat the shrink tube

The exception is the ground that gets put on the grounding screw.

Step 6: Test and Enjoy

That's it, for just a few bucks, you can now legally use a small trailer with your car's accessory hitch.

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    2 years ago

    Hey will this 5 wire into 4 wire converter box work with any small vehicle for a small utility trailer?

    I have a 2011 VW Golf wagon 2.0 diesel


    Question 4 years ago on Introduction

    Car related, but I am adapting a bicycle trailer to pull behind my Mobility Scooter. Now, the scooter has the full suite of lights, and I would like to carry that on to the trailer. I have already bought LED lights, the holes for the lights and wiring have been drilled, the wires fished in and set to attach lights, and so on.

    The issue I have is that I have to use a adapter fro 5-wire to 4-wire, so I bought a Reese kit (about $35Cdn), and when I got home, it says that the adapter works only for a "Negative Ground" vehicle.

    Well, the scooter, being made in Britain, is a Positive Ground, and there ain't no such thing as a "Positive Ground 5-wire to 4-wire trailer adapter" on this planet (according to Bing, Google and even Duck, Duck, Go!)

    So, even though I understand enough about electricity to know that it doesn't (normally) matter which polarity the Ground is, current always flows Neg to Pos. However, I need to know if I hook this up, but using the (white) Ground as Positive instead of Negative, will damage the adapter?

    I can read "some" circuit diagrams, but am not sure if the polarity change will work with the adapter, and I don't want to hook it up just to fry it.

    Oh, yeah, I forgot - the scooter and associated electronics on board, are all 24v as well, so the trailer lights will be 24v (not a problem as the LED's are rated 10-30v), and I have already worked out the polarity issue with the LED's, so I know which one is connected to the frame (Black, in this case) and the (white) lead connects to the coloured wires inside the frame tubes.

    Can you tell me if I stand a chance of frying the adapter if I go ahead and connect "like normal" knowing that the ground is Positive and not Negative? Does it really matter? Heh, I don't want to waste almost $40 for nothing - that's like 15 cups of coffee at Second Cup!


    Edwin "scooter boy" Wityshyn
    Fort Saskatchewan, Alberta


    4 years ago

    most states allow you to use a trailer with or without lights as long as you are done trailering before the sun goes down.