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4-Way Wishbone Trailer Wiring Harness with 42" Ground Wire - 40' Long

4-Way Wishbone Trailer Wiring Harness with 42" Ground Wire - 40' Long

Item # A40W42B
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Wiring A40W42B - 4 Flat - Optronics
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This wiring harness provides a simple, efficient method for rewiring your trailer with a 4-pole flat trailer-end connector. The wishbone-style harness has an extra tail light wire that makes it easier to wire your trailer. Lowest Prices for the best wiring from Optronics. 4-Way Wishbone Trailer Wiring Harness with 42" Ground Wire - 40' Long part number A40W42B can be ordered online at etrailer.com or call 800-298-1624 for expert service.
  • All Info
  • Reviews (20)
  • Q & A (16)
  • Photos
  • Why etrailer?

Optronics Wiring - A40W42B

  • Trailer Connectors
  • Trailer End Connector
  • 4 Flat
  • Plug and Lead
  • Optronics
  • 31 Feet and Longer

This wiring harness provides a simple, efficient method for rewiring your trailer with a 4-pole flat trailer-end connector. The wishbone-style harness has an extra tail light wire that makes it easier to wire your trailer.


  • Provides simple, efficient method for rewiring your trailer with a 4-pole flat trailer-end connector
  • Wishbone design - Y-harness contains an additional brown tail light wire
    • Left-side tail light wire is bundled with the yellow left turn/brake wire
    • Right-side tail light wire is bundled with the green right turn/brake wire
  • 4-Pole flat trailer end connector to connect to your towed vehicle
  • 20-Gauge wire
  • Wire colors:
    • White wire for ground
    • Brown wires for tail lights
    • Yellow wire for driver's-side stop/turn
    • Green wire for passenger's-side stop/turn


  • Wire length: 40'
  • Ground wire length: 42"

Because the brake signal is sent along the same wire as each turn signal, the tail light signal is the only one that needs to be run along both sides of a trailer. This wishbone-style harness (or Y-harness) contains 2 separate tail light wires (brown wires): one bundled with the left turn and brake wire (yellow wire) and one bundled with the right turn and brake wire (green wire). It's the most efficient way to wire your trailer.

A40W42B Optronics 40 Foot 4-Way Wishbone Style Trailer Wiring Harness with 42 Inch Ground Wire

California residents: click here

Customer Reviews

4-Way Wishbone Trailer Wiring Harness with 42" Ground Wire - 40' Long - A40W42B

Average Customer Rating:  5.0 out of 5 stars   (20 Customer Reviews)

This wiring harness provides a simple, efficient method for rewiring your trailer with a 4-pole flat trailer-end connector. The wishbone-style harness has an extra tail light wire that makes it easier to wire your trailer.

- A40W42B

Fast delivery, good product. I had a hard time finding a wiring harness long enough for my boat trailer, and the 42" ground wire setup had a really nice setup.

- A40W42B

Perfect wiring for my trailer

- A40W42B

Just what I needed to rewire an old boat trailer while I was replacing all the lights with new waterproof LED lighting.

- A40W42B

Ordered what i need and got it quickly

- A40W42B

My trailer takes a beating year after year. Replaced all my lights on my utility trailer last year. Lights still look new and all work even sitting in extreme weather.

- A40W42B

Installed this on a 5x8 enclosed trailer. It was long enough to go down both sides and up the rear and all the way to the upper front marker lights. Cheaper than Amazon too.

- A40W42B

I have a 24' pontoon trailer. The harness was plenty long to reach all the way to the tail lights, I especially like the protection around the harness at the connector end. The trailer has been in and out of the water a few times with no consequences. I would recommend this harness for any one who need to rewire their trailer for a very reasonable cost.

- A40W42B

- A40W42B

Placed order and within a few days I was able to wire trailer without no problems. They are great people to talk to and help with any questions I had regarding to wiring.

- A40W42B

Great harness! Worked as expected! Thank you!

- A40W42B

Amazing service, exactly what they said, speedy service and delivery. WOW

- A40W42B

Made wiring my trailer a breeze!

- A40W42B

Great product. Exactly what I wanted.

- A40W42B

Quality product, easy installation.

- A40W42B

Just what the doctor ordered!

- A40W42B

Great price, order shipped same day, part worked perfectly. Can't ask any more than that.

- A40W42B

this was what I was lookin for there was no surprises.

- A40W42B

Fast service and product was as expecte d.

- A40W42B

Easy install works perfect

- A40W42B

Excellent and super fast shipping


Ask the Experts about this Optronics Wiring

  • Recommended Wishbone 4-Way Trailer Wiring Harness with 14 Gauge Wire
    I do have an option for you but none of our trailer wiring harnesses we offer come with that heavy duty of a wire. The only 4-way wishbone kit we offer 40' long is the 4-Way Wishbone Trailer Wiring Harness part # A40W42B which has 20 gauge wire. I am not sure why you would need that heavy duty of a wire for tail lights but you can make your own harness using the Jacketed 6-Wire part # DW04906-1 (per foot) and the Quick Fix 4 Pole Flat Trailer End part # HMC20031. The extra wire in the above...
    view full answer...
  • 40 Foot Long 4-Way Wiring Harness Recommendation
    For a 40 foot long section of 4-way wiring that includes the connector we have the part # A40W42B. I attached a review video for this for you to check out as well.
    view full answer...
  • Troubleshooting Trailer Wiring For Double Towing Camper and Motorcycle Trailer Behind 2016 Tundra
    It sounds like you want to tow 2 trailers and need the wiring for both. It is completely possible to do that as long as your truck can handle the total weight of the two loaded trailers. For the wiring, there are a couple of different options. You can run an extremely long harness like you mentioned that will be a simple customized length or you can tap into your trailer wires to create a harness at the back of your camping trailer. My recommendation is to do the latter. To...
    view full answer...
  • Recommended 4-Way Wishbone Trailer Wiring Harness for Boat Trailer
    Based on color, I can't tell if you have functions that should be tied together or not. I've attached an article on trailer wiring to assist as you test functions. I recommend using a 12V battery to test the function of each wire on your trailer. When that function lights up, you will know what that wire function is and which pin on your vehicle side wiring that it should connect to. I recommend using the 4-way Wishbone Trailer wiring harness # A40W42B along with the Deka Heat Shrink...
    view full answer...
  • 40' Wishbone Trailer Wiring With A Ground Wire Running Down Both Sides
    We do not offer wishbone wiring that has a ground wire going down both sides. You really don't need it because the ground wire is for the plug itself not the lights and you want the ground close to the plug. I recommend using the 4-Way Wishbone Trailer Wiring Harness Item # A40W42B.
    view full answer...
  • Trailer Wiring Connector for Triton 8x10 Aluminum Snowmobile Trailer
    For your wiring, you will need to test your wire functions to know which one connects where. I recommend using a 12v battery and simply connecting the negative terminal to the trailer frame and then connecting the power to each wire at the connector individually to determine function. It is not uncommon to have two wires running to one function location for each side of the trailer so there aren't any jumper wires across the frame needed from side to side. I've attached an article...
    view full answer...
    Image 1 for Image 2 for
  • 4-Pole Wishbone Harness for Running Light Connection
    Connecting the brown running light wire in standard 4-pole wiring to both tail lights will require a jumper wire from one side of the trailer to the other, or you can install what's called a wishbone harness that has dual brown running light wires, one for each side of the trailer. We have several of these trailer-side 4-pole wishbones, such as 40-foot # A40W42B, 35-foot # A35W42B, 25-foot # A10-4225VP and 20-foot # A20WB. I do recommend using a wishbone harness instead of a spliced...
    view full answer...
  • Recommended 4-Way Trailer Wiring Harness For Adding New LED Lights To Vintage Trailer
    To wire the 4 clearance lights # MCL0040ABB, license plate light # M298C, and tail lights # STL190RB to your 12' long 75" wide vintage trailer, I recommend starting the 4-Way Wishbone Trailer Wiring Harness with 42" Ground Wire # A40W42B which is 40' long meaning you will have enough extra wire to connect all of the lights you are planning on adding. The # A40W42B will install as follows: Brown wire to each of the yellow wire on each of the # MCL0040ABB, to the brown wire on the #...
    view full answer...
  • Troubleshooting Wiring Issue On Harbor Freight Trailer
    It does sound like you have done the ground wiring well. I recommend putting a circuit tester like # PTW2993 on multiple spots as you test wiring from the truck to the trailer lights. Starting at the truck with the trailer disconnected, test each function of your 4-way. If all checks out, add the extension and check each function. If that checks out, then test right behind the trailer wiring connection to that extension. If everything works at this point, you have 100 isolated the...
    view full answer...
  • What Does Some Trailer Wiring Have 2 Brown Tail Light Wires
    Common 4-pole trailer wiring like # HM38138 uses one wire, the brown one, for the running lights on both sides of the trailer. Sometimes what is called a 'wishbone' harness is installed on the trailer and this simply uses two separate running light wires - brown with yellow for left and brown with green for right - to make it easier to connect to the trailer lights since that way no jumper wire will be needed to cross from one side to the other. Some examples of wishbone style 4-pole...
    view full answer...
    Image 1 for
  • What Wire Gauge Should be Used to Install Peterson Trailer Tail Light # 429800
    Trailer light wiring is usually 14 to 18 gauge. But we have 4-Way flat trailer wiring harnesses all ready to go. I have included a link to the options for you. We have some as short as 1 foot long all the way up to the 40 foot long # A40W42B.
    view full answer...
  • How to Make 4-Way Extension Over 35 Feet in Length
    For a 40 foot long section of 4-way wiring that includes the connector we have the part # A40W42B. The for a 4-way to wire to the other end of the harness we have the part # HM48030 which would complete your 4-way extension.
    view full answer...
  • Replacement 4-Way Flat Wiring Harness for 1959 Hawthorne Trailer.
    If your current wiring is still working a Test Light # PTW2993 can be used to help determine what color wire controls what on your current wiring at the taillights and any marker lights along the way. I have also linked an article on 4-way wiring below for you to reference as well. Otherwise you can use a 12 volt battery grounded to your trailer chassis to test the individual wiring by providing battery positive to each wire to determine what each wire powers. I recommend replacing...
    view full answer...
  • Recommended LED Trailer Light Kit with 4-Way for Longer Trailer
    The Aero Pro LED Trailer Light Kit part # TLL36RK is an excellent option to add LED lights to a trailer as it comes with everything you will need so you don't have to worry about combining items yourself and ensuring they are all compatible. The Aero Pro LED Trailer Light Kit comes with a 25' long 4-way harness. If you need more length you can extend the wires with part # 16-4B-1. We also have the 4-Way Wishbone Trailer Wiring Harness part # A40W42B which is 40' long. It really just comes...
    view full answer...
  • 4-Way Extension Recommendation for Rear of Ford F-250 For Towing Second Trailer
    The Curt T-Connector part # C55774 that you referenced would be the first component you'd want. This will give you a 7-way and 4-way active at the rear of your truck. This will allow you to run a 4-way extension from the truck to the second trailer to activate the turn signals. For an extension you could use the part # A40W42B which is a 40 foot section of 4-way wiring that includes a male 4-way and then to the other end you could attach the part # HM48030 and you'd be set.
    view full answer...
  • Converter To Provide Separate Brake And Turn Signals Functions On Trailer From A Standard 4 Pole
    On standard trailer wiring the right and left wires run as a combined signal. So the brake and turn signal for one side will be combined and the bulb itself acts as a brake and turn signal depending on the signal current. Since you are wanting to use separate brake and turn signals from a standard combined wired 4 pole plug you are going to need the Vehicle to Vehicle Tail Light Converter Item # 118158 as well as the 4-Way Wishbone Trailer Wiring Harness with 42" Ground Wire - 40' Long...
    view full answer...

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Info for this part was:

Employee Leah B
Edited by:
Leah B
Employee Jameson C
Expert Research:
Jameson C
Employee Cole B
Installed by:
Cole B
Employee Amy B
Written by:
Amy B
Employee Chris R
Video Edited:
Chris R
Employee Shane H
Test Fit:
Shane H
Employee Jonathan Y
Video by:
Jonathan Y

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