Right side taillight not working?

Tiny
LOOSINGMYMIND
  • MEMBER
  • 2014 GMC SIERRA
  • 6.2L
  • V8
  • 4WD
  • AUTOMATIC
  • 123,000 MILES
The taillight on the right rear is not working, I have everything, brake lights, signals, backup lights. Replaced the power block, Right wiring harness, just the right taillight is out, bulbs are all new, fuses seem to be good, power at relay and fuses, headlights have started to go out. I turn off truck restart they will stay on, but no taillight, I have upgraded my headlights to LED bulbs long ago and just recently have started cutting out. Do you think they could be linked and causing this taillight problem?
Monday, March 4th, 2024 AT 8:11 PM

23 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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Do you have power at the PRK LMP RT fuse? 15 amp under the hood fuse panel? The left and right are connected so if the left works that is a good sign.

Here is a guide to help you check for power at the fuse then we can go from there:

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Here is the fuse and wiring diagrams for the system so you can see how it works. I don't like the aftermarket headlights; I would swap those out for the factory lights. If you still have problems with the headlight, please start a new thread for that problem. Check out the images (below). Please let us know what happens.
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Tuesday, March 5th, 2024 AT 9:58 AM
Tiny
LOOSINGMYMIND
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Yes, I have power at both fuses at that location, when I switch on the park lights, I can also hear the relay clicking, I have replaced both of them with new just to make sure.
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Tuesday, March 5th, 2024 AT 10:09 AM
Tiny
LOOSINGMYMIND
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It’s the right side that is not working, passenger side of truck, you always go by when you are sitting in vehicle correct. Just want to make sure we are on the same page.
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Tuesday, March 5th, 2024 AT 10:13 AM
Tiny
KEN L
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Yep, the right side, here is the rest of the parking light wiring diagrams so you can see the left and right side are connected. If one works so should the other. Do you have power at the gry/brn wire when the parking lights are on? Check out the images (below). Let us know how it goes.
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Tuesday, March 5th, 2024 AT 11:12 AM
Tiny
LOOSINGMYMIND
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  • 16 POSTS
Do you mean gray and brown at the taillight? Should I check it at the bulb socket?
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Tuesday, March 5th, 2024 AT 11:34 AM
Tiny
LOOSINGMYMIND
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No power at marker lamp when switch is on.
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Tuesday, March 5th, 2024 AT 11:51 AM
Tiny
LOOSINGMYMIND
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No power at marker light socket.
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Tuesday, March 5th, 2024 AT 11:54 AM
Tiny
KEN L
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So, you have power at the fuse "PRK LMP RT" and there is no power at the gray and brown down by the socket, correct? As you can see by the wiring diagrams there is only a rear body junction block that stands between the fuse panel and light bulb socket, I would get under the truck and trace the rear wiring harness to look for damage to the harness and also see if you can find the junction block and check that for damage.
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Wednesday, March 6th, 2024 AT 8:28 AM
Tiny
LOOSINGMYMIND
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Okay, the power block in the back by spare tire, I have changed, so it’s going to be from that block to the front of the truck, it’s not going to be easy but I’ll get under it on drivers side and look for some kind of damage in any harnesses I can see. Thank you
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Wednesday, March 6th, 2024 AT 9:10 AM
Tiny
KEN L
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Sounds good, please let me know what you find.
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Wednesday, March 6th, 2024 AT 10:38 AM
Tiny
LOOSINGMYMIND
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I have a question, if the power from the relay and both fuses go to the taillights, does that power also go to the license plate light sockets, both of them bulbs have power. That would tell me that the power supply from the fuses is making its way to the power block or at least to the back of the truck. I guess I’ll have to peel back some more wire covering to see if the damage is at that location near the power block.
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Wednesday, March 6th, 2024 AT 9:36 PM
Tiny
KEN L
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No, the BCM controls the license plate light. Here are the complete wiring diagrams so you can see how the system works. I would probe the gry/brn wire at the right taillight as it is going into the connector. Check out the images (below). Let us know what you find.
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Thursday, March 7th, 2024 AT 9:40 AM
Tiny
LOOSINGMYMIND
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Hi, I removed the power block, disconnected all the plugs, used a test light checked the power at the harness coming from the front of the truck, has full power, left taillight at the block with power harness plugged back in, has full power at the power block, the right hand tail light only has about half power when tested with the test light at the block, it’s the only plug socket in the block that doesn’t have full power. Would this indicate bad ground coming from front of truck, headlight switch, fuse panel, battery ground to firewall or block. I have removed three grounds cleaned and reconnected at back of truck, each side of frame on the front of truck.
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Thursday, March 7th, 2024 AT 11:00 AM
Tiny
LOOSINGMYMIND
  • MEMBER
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Nothing has changed, testing at end of taillight harness at bulb locations, side marker no power. At power block with taillight harness removed two very weak signals dim light, every other plug has full power.
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Thursday, March 7th, 2024 AT 1:28 PM
Tiny
KEN L
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It sounds like you have found the problem, so you have power at the gry/brn coming into the junction block but not coming out of it, just runt the gry/brn directly to the right taillight and reconnected it effectivity bypassing the shorted junction block.
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Thursday, March 7th, 2024 AT 4:31 PM
Tiny
LOOSINGMYMIND
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I would agree with you, but the block is new. I replaced it and the right taillight harness at the same time, about two weeks ago. Unless it is screwed as well, I just can’t get my head around it being toast like the old one.
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Thursday, March 7th, 2024 AT 4:55 PM
Tiny
LOOSINGMYMIND
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I turn on the park lights, ground out the test light to the frame and find the power in that harness coming to the power block, then I take the grounding side of the test light and touch every one of the other pins and I get a light except one. I think I’m still missing something.
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Thursday, March 7th, 2024 AT 5:13 PM
Tiny
LOOSINGMYMIND
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I should have found the ground and touched the power to see if I had a light, I think I’m not getting a good ground in that harness from the front of the truck. I’m running out of ideas.
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Thursday, March 7th, 2024 AT 5:21 PM
Tiny
KEN L
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Black is ground, connect the test light to power to see if it lights up.
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Friday, March 8th, 2024 AT 9:06 AM
Tiny
LOOSINGMYMIND
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Hi guys, I was wrong, I put back in the old harness, cut the ground and added my own, plugged it back into power block, turned on park lights didn’t work. Then I added a power wire, cut the power lead brown and added power from my trailer plug to the harness, turned on the park lights and now have taillights. I don’t know what changed, I tested what I thought was the power coming into the power block, one pin has full power, the power block is new, but I guess it could be faulty, are there two powers coming from the fuse panel to the back, when I tested power block I had full power and a weak power in the harness and also the socket when I plug in the front power harness to the power block.
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Saturday, March 9th, 2024 AT 10:39 AM

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