Troubleshooting Horse Trailer Interior Lights Not Functioning

Updated 11/18/2025 | Published 11/19/2014

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T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

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Question:

I have a new horse trailer - all lights work except the interior lights. I am hooking it up to a seven prong outlet on a 2010 gmc sierra. what should I check and what lights need to be activated on the truck for these to work?

asked by: Graham

Expert Reply:

The interior lights of your trailer would be powered by the 12 volt accessory circuit on the 7-way of your 2010 GMC Sierra. You will want to check the 1:00 position of the 7-way on your truck with a circuit tester like part # PTW2993 to verify if there is voltage on this pin. If there isn't voltage on this pin you are most likely missing the 40 amp fuse in the power distribution box and/or the circuit under the hood hasn't been completed.

For more info on how to complete the circuit under your head check out the FAQ article I attached to the right. Scroll down to the Finalizing Connections for the Brake Controller Heading and then check out step 3.

If there was voltage on the circuit of your 7-way on your truck you will want to check the trailer's connector for corrosion and clean as necessary and then trace the circuit to your interior lights fixing any obvious signs of a short. Also, make sure the assemblies are grounded to a clean metal surface.

expert reply by: Jameson C

[email protected]

2/25/2020

Where is the 40 amp fuse located under the hood that runs your interior horse trailer lights. Im having same problem with my 2008 dodge 3500 ram mega cab dully hitched to my trailer. All other lights work :

Chris R.

2/26/2020

Assuming the interior lights on your horse trailer are powered by the 12V circuit on the 7-Way, you will want to check for the pink 30 amp fuse located in the # 19 position under the hood of your 2008 Ram 3500. This fuse is in charge of the trailer tow 12V charge.

Joyce M.

8/5/2021

@ChrisR Where is the 40 amp fuse on 2011 Silverado 2500 exactly. I know under hood, but can’t figure out which fuse I need to check.

Victoria B.

8/12/2021

@JoyceM The fuse location you are looking for is in what Chevy calls the J-Case Fuses box, in location 68. I found this listed in the owner's manual on pg 10-51. They call this location Stud 1.

See All (4) Replies to [email protected] ∨

Mark L.

5/27/2020

So, I am having the same problem. (2017 Ford f150) All Exterior lights were fine, the switched interior lights are not working. One interesting piece is that While doing some maintenance on the trailer, I pulled the battery for the E-brake system. (I knew it was older) and when I disconnected the battery the interior lights would work. I assumed a bad battery so I replaced it. The old battery disconnected only read .37 volts. The new battery before installing read 12.47 volts. After reconnecting the new battery, interior lights will NOT work again ???????? (Trailer was not physically hooked to truck, just 7-pin wire plug was plugged into truck)

Chris R.

5/29/2020

That is strange. So the interior lights turned on after disconnecting the battery? I'm trying to understand what they would have been powered by if that's the case. I would try as best you can to inspect the wiring from the interior lights all the way to the battery - look for any exposed or pinched areas that need to be replaced. I'm assuming there's a short somewhere on this circuit that just needs to be found.

Mark L.

5/29/2020

@ChrisR With the battery disconnected the lights were receiving their power from being plugged into the truck obviously. I am just trying to figure out why they will not work with the battery plugged in and when you disconnect the battery they work. NOTE: I do know there is some type of module inline on the positive(power) wire in the battery box. could that be some type of switching relay ??

Chris R.

6/1/2020

@MarkL Ah that makes sense (well, more sense at least). So your trailer's interior lights are connected to the 12V output circuit coming from the truck - and that same circuit is what is maintaining the charge on your trailer battery during travel. I would absolutely try and trace the wiring going from the trailer battery all the way up to the 7-Way. There's most likely some exposed wiring along the way that's shorting out and killing this circuit when connected. Let me know what you find.

Ajm

6/16/2020

I have a 2005 Chevy Silverado and just bought a 2 horse bumper pull. I previously towed a goose neck with the 7 way plug. When I hooked to the 2 horse trailer up, all running lights and break lights work but the inside trailer and tack room lights do not work. What can I do to get these to work?

Chris R.

6/19/2020

As mentioned above, the interior lights are probably powered by the 12V circuit on the 7-Way, so you'll need to make sure that pin is active on your Silverado. If not, you likely just need to replace the appropriate fuse under the hood (check our owner's manual for the right location). Let me know if this doesn't work and we'll keep troubleshooting.

Misty S.

7/15/2020

Interior light will not work with my 2012 ford raptor. Brand new trailer. All outside lights are working. Where do I need to look?

Chris R.

7/20/2020

Assuming the interior lights power through the 12V circuit from the Raptor's 7-way, you'll definitely want to check under the hood for a blown fuse. Look in your truck's owner's manual for the appropriate fuse location (it will probably be listed as "trailer charge" or something related.

Shaun K.

10/19/2025

@ChrisR I have the same issue. 2010 F150. The 2 fuses in the engine bay were not factory installed for the brake controller wiring.. I added them, it made the trailer accessory lights come on. However, when I had the running lights on and flipped the accessory switch, it blew the fuse to everything including the brake controller. Any suggestions?

Robin H.

10/21/2025

@ShaunK make sure you have the proper fuses. Other than that I wouldn't expect it to actually blow the fuses so there could be an issue with the wiring. Are you using a custom wiring harness for the brake controller or was it spliced in? If you spliced it make sure it is properly installed.

See All (5) Replies to Misty S. ∨

Mark

11/18/2025

I am have the same problem all lights work and not the interior. Looked up the fuse box nothing really named trailer charge? I have a 2019 gmc 1500

Jameson C.

11/18/2025

@Mark There are some connections that need to be made under the hood of the truck in order for the 12 volt accessory circuit to be active. Check out the link below where you can see what all you'll need to do.

Helpful Links

faq-brake-control-gmc-sierra-chevy-silverado.aspx