How Trailer Wiring Works

Boat trailer
All these folks need is a tow vehicle and some trailer wiring smarts and they'll be fishing in no time. See more truck pictures.
Marc Debnam/Getty Images

You bought your nifty sports utility vehicle (SUV) and your boat and trailer, and you're ready for a weekend on the lake. The wife has the food packed and ready, and the kids are slathered in sunscreen, eager for some water-skiing. All you need to accomplish as the head mechanic in the family is to hook your SUV to your boat trailer and hit the road. All goes well with the hookup, but when it comes time to wire your trailer to your truck, things get a little iffy. Your trailer wiring plug doesn't match the one on your vehicle. The wife and kids stare daggers into your head as you curse your way through hours of agonizing wiring mishaps. Thanks, Dad.

Each state has its own set of towing laws and regulations, from size limits to weight restrictions. The one common denominator for each state in the U.S. is that the trailer must be wired to the vehicle. Just like your car or truck's brake and signal lights must be in working order, so should your trailer's lights. This is accomplished by wiring the vehicle to the trailer.


Many times, trailer wiring is as easy as inserting the trailer plug into your vehicle's pre-rigged system. In fact, most vehicles, especially those meant for towing, come prewired and ready to roll. Whether your vehicle is pre-rigged or not, you'll have to go through the necessary steps to make sure your trailer is wired to your car. Besides being the law, it's also common sense. You'll want the cars behind you in traffic to see every brake tap and turn signal on your trailer, just like they would on your car.

Read on to find out more about trailer electronics. How can you wire your trailer to your tow vehicle to ensure towing safety on your journey to your recreational destination?



Trailer Lighting

Trailer hauling motorcyles
Even small trailers like this one for towing motorcycles need to be wired to your vehicle.
Ryan McVay/Getty Images

If you're going to tow a trailer, you need your trailer lights up and running. Most trailers have three circuits that you'll connect to your vehicle. One operates the taillights, one runs the left brake light and the last one runs the right brake light. When it's time to turn, your brake lights will flash to let everyone know which way you're going. Obviously, trailer lighting is essential to safe towing.

You also need a ground wire. This provides a conducting path which is independent of the normal current-carrying path -- a fancy way of saying that it prevents the system from shorting out due to an electrical surge. So this means your trailer wiring plug will need at least four contacts. The good news is that the standard connector for trailers uses four pins, plus the ground. These plugs are typically flat, with the pins set in a row. Easy enough, right?


Some of the other common styles of connectors use five, six or seven pins. These plugs are usually round and plug in much like an electrical socket would. But if you only need the brake lights and taillights hooked up, why would you ever need more than four pins? It's because larger trailers will occasionally have a separate circuit for running lights on the sides and front of the trailer. Your extra pins will operate these lights. Some trailers, like ones that carry cars or horses may have interior lights powered by the tow vehicle's battery. This can account for the need for another circuit as well.

If your vehicle is pre-rigged with a four-pin plug and your trailer is ready to go with a four-pin plug, all you need to do is plug it in and test it out by doing the following:

  • Turn on your vehicle lights and make sure the trailer taillights come on.
  • Tap your brakes to make sure the trailer's brake lights work.
  • Test each turn indicator and make sure the trailer complies.

That's all there is to it in the best case scenario. Another situation might find you with a seven-pin connector on your tow vehicle and a four-pin connector on your trailer. This is when you need to adapt -- literally. Go to your local auto parts store and ask the specialist which adapter you need, if you don't feel comfortable picking it out yourself. Chances are it will be as easy as locating something with a label that reads "Seven-pin to four-pin trailer wiring adapter."

­Read on to find out about buying trailer wiring kits whe­n your vehicle isn't pre-rigged.


Trailer Wire Identification

Wiring harness
Standard four-pin wiring harness
HSW 2008

Let's say that your vehicle isn't pre-rigged for towing. Don't worry, you can still take your boat to the lake for the weekend. You just need to buy a trailer wiring kit and do it yourself or have a professional do it for you.

The easiest way to find the wiring kit you need is to order it on the Internet or go to an auto parts store that has a nice selection of wiring systems. The kit will be specific to your vehicle make, model and year. These days' kits take most of the brain work out of it, and you won't need to wire anything yourself. That means no splicing, soldering or even taping of wires. What you'll get in the kit will most likely be a T-harness or T-connector. The T-harness has two plugs, one for each taillight. These plugs are connected to each other and feed into a single plug that hooks to your trailer. Each side of the T-harness will plug into your taillight, and the original plug will then go into the newly installed T-harness.


Once you have the right T-connector for your vehicle, it's simply a matter of plugging it all in. What you want to do here is access the rear taillights of your vehicle from the inside. If you have a wagon, you can do so by simply looking under your trunk. Lift up the carpet in the rear and you'll find a storage area where your spare tire and jack are most likely kept. There may also be a piece of molded plastic that shields the taillight from the rest of the trunk. You need to remove this as well -- it most likely pops out and snaps back into place pretty easily. Some SUVs won't have access to the light from the trunk, so you'll have to remove the entire taillight housing with a screwdriver.

Once you have access to the taillights, you'll notice the wiring plug leading into it. This comes directly from your car's electrical system. All you need to do is unplug it from the light, plug your T-harness in and then plug the car's original set of wires into the newly installed trailer harness. Do the same thing on the other side, and you'll be wired for towing your trailer. Then simply attach the ground wire to some metal. It will probably be a ground with a metal loop at the end. Just drill a hole into the metal base of your trunk and drive a screw through the loop to attach it to your car.

The plug that you'll attach to your trailer will have several feet of wire. You can just keep the plug and wire stowed under your trunk storage space alongside your spare and jack until you need it for towing. When that time comes, simply pull it out and close the trunk on top of the wire -- you won't do any damage to the system.

If you find that you have a car that's too old or a trailer that has a mess of wires, you may need to bring your vehicle in for some help from a mechanic. Otherwise, it's as easy as plug and tow.


Trailer Plugs and Sockets

When it comes to trailer wiring and harness plugs, there are several to choose from in a wide variety of sizes and configurations. The standard trailer plug and socket design is a color coded system using four wires. In this case, the brown wire is used for the tail lights, the license plate light, and the side marker lights. The yellow wire gives power to the left-hand brake light and the left-hand turn signal. There's a green wire that controls the right-hand brake and light and the right-hand turn signal. The last wire of a standard four-wire system is white -- it's used as the ground wire. Most boat trailers and small utility trailers that don't have their own brakes use this kind of harness plug, often referred to as a "flat-four."

When researching what kind of trailer plug and socket system to go with, you may hear them referred to as having a certain amount of "poles." This term is synonymous with the number of wires. Following are some of the other wiring systems you may want to consider, depending on your needs:


Five-wire systems are the same as the four-wire system, right down to the wire color and corresponding function. The one difference is the additional wire. The fifth wire for these units is blue and is typically used to control things like hydraulic disc brakes or additional auxiliary outlets. This can include interior trailer lights or extra side lights.

Six-wire systems are also the same as the four-wire models but they have, you guessed it, two extra wires. In this case there's the additional blue wire that would serve the same purpose as the one for the five-wire system. There's also a red wire that acts as a 12-volt feed. This can operate everything from a cigarette lighter to a car alarm system.

The seven-wire system corresponds exactly to the six-wire systems plus one bonus pole that's generally used to power interior and exterior lighting. You'll often use a seven-wire setup for a camper trailer, RV or cargo trailer.

As with any kind of auto wiring, trailer plugs and sockets can be a little confusing for the novice trailer tower. Before you go and plunk down some hard-earned cash for a wiring harness, talk to an expert. Your mechanic or your local auto parts store employee may be able to give you the advice you need. It also may be a good idea to get some information from some wiring manufacturer Web sites before you start asking questions.

For more information on towing your boat and whatever else you'd like to link to your vehicle, visit the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links


  • "How do I troubleshoot trailer wiring problems?" 2008.
  • "How Plug-In Simple! Trailer Wiring Works" 2008.
  • "How to wire a car for trailer lights.", 2008.
  • "Trailer Wiring Diagrams." 2008.
  • "Trailer Wiring for the Do-It-Yourselfer." 2008.
  • Allen, Mike. "Saturday Mechanic: Wiring Your Trailer Hitch." February 2004.