How to Wire a 7-Pin Trailer Plug

by Robert Moore

Wiring a 7-pin plug on your truck can be a little intimidating when you are looking at it from outside the box. There is a lot wiring that you have to tie into your truck's wiring harness, but it is easier to do than it seems. All makes and models are designed differently, so you’ll need to have a service or repair manual that includes wiring diagrams for your specific make and model. The following instructions describe the wiring of a basic, traditional plug that is commonly used on pickup trucks. If you’re installing a plug designed for an RV or a heavy-duty truck, refer to the last section for specific color codes.

Tip

  • If you are wiring a harness on a vehicle that has an existing warranty, professional installation of the plug is recommended. Wiring an aftermarket plug as opposed to an original equipment manufacturer plug may void the existing warranty on your truck's electrical system.

Making wire connections

Almost every wire for your 7-pin plug must be spliced into the wiring for your truck. To properly make the connection use the following steps:

Trim back the insulation on the existing wire.

Use wire trimmers to reveal about 1 inch of copper wiring.

Trim and connect a new wire to the existing wire.

Trim about an inch off the insulation of the new wire. Hold the new wire parallel to the existing wire so that the exposed strands of each wire are next to each other. Twist the new wire around the strands of the existing wire.

Solder the connection.

Hold a soldering iron against the bottom of the wire joint. Hold solder against the top of the joint. Wait for the solder to melt and wick through the wire strands. Remove the soldering iron from the wire joint.

Brush a thick coat of liquid tape over the wire joint.

Apply the liquid tape to the joint. Ensure that you cover all bare wiring. Once the liquid tape dries, apply a second coat of tape to completely seal the wire joint.

Installing the wiring

Tip

  • To see a visual representation of a 7-wire plug and a diagram of how the wires connect to it, view a diagram available on etrailer.com.

Install a memory saver and disconnect the battery.

Set up a memory saver according to its manufacturer’s directions. Plug the memory saver into your truck's 12-volt accessory socket. Loosen the negative battery cable with a battery wrench, and then remove the negative battery cable from the negative battery terminal. Isolate the negative cable from the negative battery cable.

Tip

  • A memory saver maintains a constant power supply to your vehicle's radio and computer systems when the battery is disconnected. If you choose not to use one, ensure that you have your radio's security code in hand, as you will need it for the radio to operate after you reconnect the battery. Without a memory saver, your engine will idle differently and the transmission will shift harshly. This is normal and everything should return to normal after your vehicle's computer relearns proper idle parameters and shift points.

Tap into a fused, constant 12-volt source.

Look at the wiring diagram for your truck's ignition switch. Determine which color wire supplies constant power to the ignition switch. Locate this wire where it comes out of the interior fuse box. Connect the black wire for your 7-wire plug to this wire.

Connect to the brake controller and route the black and blue wires.

Find the blue wire that comes off of your electronic brake controller. Attach the blue wire that came with your 7-wire plug to this blue wire. Locate the rubber grommet that surrounds the interior wiring harness where it feeds through the firewall. Feed the black wire and the blue wire through the grommet and route it to the rear of the truck. Secure the wires to the wiring harness that runs along the frame rail with a zip strip every 5 feet.

Connect to the left-turn signal and reverse lights.

Drop the tailgate and find the mounting screws for the left taillight. Remove the two mounting screws, then pull the taillight away from its hole in the left bed panel. Disconnect the wiring from the taillight and set the taillight assembly aside. Refer to the wiring diagram for your truck and determine which wire in the harness for the left taillight assembly controls the left-turn signal and brake. Also, determine which wire powers the reverse light. Attach the yellow wire to the wire for the left-turn and brake light. Attach the purple wire to the wire that powers the reverse light. Once the liquid tape is dry, zip-strip the wires to the existing wiring harness, and then feed them through the hole in the left panel of the bed. Allow the wires to rest on the ground for now.

Connect to the right-turn signal and the running lights.

Remove the right taillight assembly and disconnect the wiring harness. Identify the power supply wire for the running lights and the right-turn signal. Connect the brown wire to the wire for the running lights and connect the dark green wire to the wire for the right-turn signal. Wait for the liquid tape to dry, zip-strip the wires to the existing wiring harness and feed them through the hole in the right bed panel. Allow them to rest on the ground for now.

Make the ground connection.

Look at the wiring diagram for your truck and determine the designation given to the ground location for the taillights. The label starts with a “G” followed by a three-digit number like G104. Find the location-identification section in the manual for your truck to determine exactly where the ground connects to the truck. Remove the bolt for the ground connection. Slide the eyelet for the white wire onto the bolt along with the stock ground wires, and then install the bolt. Tighten the bolt to the specifications given in your manual.

Attach all of the wires to 7-wire plug.

Hold the plug so that you are looking at it from the rear with the hinge for the front access door on top. Loosen the screws for the six outside terminals. Hold the purple wire between the mounting plate and the center terminal and hold the yellow wire between the mounting plate and left-center terminal. Tighten the screw at the left-center terminal. Slide the white wire between the lower-right plate and terminal, then tighten it. Attach the blue wire to the lower-left terminal and the green wire to the left-center terminal. Attach the black wire to the upper-left terminal. Ensure that all of the terminal screws are tight.

Mount the plug, secure the wires and reconnect the battery.

Mount the plug to your truck's bumper following the directions included with the plug kit. Take any remaining slack from any of the wires and secure them together with a zip strip, then secure them to the back of the bumper with a zip strip. Connect the ground cable to the negative battery terminal, then tighten it. If you have a torque wrench, tighten the negative cable to the specifications given in the manual for your truck.

RV and Commercial 7-wire plugs

If you're installing a 7-wire plug for an RV, or a commercial vehicle, the color of the wires included will be slightly different than that of the traditional 7-wire plug. If you have an RV-style plug, the wiring colors are as follows:

  • Tail and running lights - green
  • Constant 12-volt - black
  • Reverse lights - yellow
  • Right turn signal and stop - brown
  • Left turn signal and stop - red
  • Brake controller - blue
  • Ground connection - white

If you're installing a commercial-grade 7-wire plug, the wire colors are as follows:

  • Ground connection - white
  • Tail and running lights - brown
  • Clearance and side-marker lights - black
  • Auxiliary or ABS - blue
  • Left-turn signal and hazard - yellow
  • Right-turn signal and hazard - green
  • Brake lights - red

Tip

  • To see a visual representation of where the wires connect to the RV or commercial-grade plug, visit etrailer.com and view the diagram labeled "Vehicle Side 7-way."

Items you will need

About the Author

Robert Moore started writing professionally in 2002. His career started has head writer and Web designer for VFW post 1224 in Hamburg, Michigan. He has prepared business plans, proposals and grant requests. Moore is a state of Michigan-certified mechanic and is pursuing an Associate of Arts in automotive technology from Lansing Community College.