Truck Bed Cap Wiring?

15_silverado_wt · October 1, 2021

Hey Everyone,

Bought a used bed cap for my 2015 Silverado 1500 WT.

I know its probably a stretch to assume this, but is factory harnesses to plug in the wire harnesses on the truck that Im not seeing? or am I gonna have to cut and solder up my own harnesses?

From what I know, my cap was specifically for my model and generation of of silverado so I figured maybe it came with factory compliant hook ups. Figured it was worth asking.

Ive attached pictures of my wire plugs that attach to the cap's third brake light and it also has an interior dome light.

If I gotta solder my own harness on, can anyone direct me to the best way to do this for my truck?

mikeyk101 · October 2, 2021

There is no cap that I've ever heard of that mates to factory connectors. All the caps I've owned have had to be wired in. There should have been an aftermarket harness that those connectors would plug in to that gets wired into the truck. Maybe call the manufacturer of the cap and see if you can purchase it?

The purpose of those connectors are so when/if you ever wish to take cap off, it can just be disconnected and then reconnected when you put cap back on.

Edited October 2, 2021 by mikeyk101

CrawlSlow · October 2, 2021

What is the small black and red wire for? Reason asking is, I have a set of wire that look just like that and I don't know if I should just cut it off or not. I will probably never get a camper. I'm assuming it's for the camper.

mikeyk101 · October 2, 2021

10 hours ago, CrawlSlow said:

What is the small black and red wire for? Reason asking is, I have a set of wire that look just like that and I don't know if I should just cut it off or not. I will probably never get a camper. I'm assuming it's for the camper.

Its probably for some accessory. I'm assuming (which can be dangerous) that you're not the original owner. Check and see if it's powered all the time. If not, check and see if it gets power when cargo light is switched on. My guess is that it was for either bed lights at one time or maybe previous owner had a cap and it was there to power interior ceiling

light???

mikeyk101 · October 4, 2021

@15_silverado_wt Download this pdf and it should give you some helpful advice...

https://www.gmupfitter.com/wp-content/uploads/2021/05/UI-Bulletin_122b.pdf

15_silverado_wt · October 4, 2021

On 10/2/2021 at 12:15 PM, mikeyk101 said:

Its probably for some accessory. I'm assuming (which can be dangerous) that you're not the original owner. Check and see if it's powered all the time. If not, check and see if it gets power when cargo light is switched on. My guess is that it was for either bed lights at one time or maybe previous owner had a cap and it was there to power interior ceiling

light???

From what I can see it runs to the third brake light, but there is also a small dome light inside the cap as well. I assumed that wire went to the interior light and the bigger harness went to the brakes.

mikeyk101 · October 5, 2021

From what I get from that bulletin, you would need the harness normally used to have underbelly lighting. Here is one place for it but not recommending this retailer: https://www.gmpartsstore.com/oem-parts/gm-tailgate-wiring-harness-22878806

If you go to the last page, you can see that pin #1 is for the accessory lighting which would be controlled by the cargo lamp switch. Although you could just tap into an always live 12v source to power the interior cap lighting. In the same connector that this harness plugs into, pin #4 supplies power when CHMSL light is activated by brakes. The directions recommend pulling the pin from #1 and putting it in the pin #4 slot and that will give you access to a source to activate your cap brake light.

On mine, I wanted the interior light tied into the cargo lamp switch. I just leave the interior cap light switched on all the time and can control it from the drivers seat. This has come in handy many times especially when traveling with my dogs who ride back there. I also found out that while driving down the road, if for some reason I need to see back there at night, I can switch into neutral and then the cargo lamp switch will turn on the interior cap light.

If it were me doing this, I would leave the #1 pin in place to control interior cap light but add another pin and wire into the #4 pin and wire up the cap brake light into that.

mikeyk101 · October 5, 2021

Oops, I spoke too soon, I guess I confused the 14-15 with the 16-18. The #4 and #1 pin are opposite. So on the 14-15, pin #1 is brake light and #4 is the accessory light for bed lights. Sorry...

And you will still need at least one of these but might consider picking up a couple. The listing does mention free samples so you might give that a try: https://www.te.com/usa-en/product-1-968857-3.html

And then the seal which looks like it's available on Amazon:

https://www.amazon.com/Connectivity-828905-1-pcs-SINGLE-WIRE-SEAL-HOLE/dp/B00HKI1HCU/ref=mp_s_a_1_3?dchild=1&keywords=828905-1&qid=1633402099&sr=8-3

Edited October 5, 2021 by mikeyk101

Andre LaVoie · October 14, 2021

2015 GMC 2500 Base/WT Matching Leer topper

I also am needing to wire in the third brake light. It currently has no wire or anything and I don't see a factory connect on the truck (I just bought the truck). What I am thinking of doing is running the wire along the ceiling towards the front and exiting out to the truck third brake light/cargo light. There I will splice in a waterproof connector to the brake light and cargo light. That way when I want to take the topper off no problem, but I also want to be able to turn the lights (that I'm adding to the ceiling) in the topper with the cargo light button. Seems like the shortest solution.

Taking criticism and solutions.

MikeBMW · October 14, 2021

GMC 2500? Maybe try here?
https://www.gm-trucks.com/forums/forum/174-2015-2019-silverado-hd-sierra-hd/

Harry S. Stamper · October 28, 2021

https://truckoutfittersplus.com/truck_topper_parts/Topper-Wiring-Harness/Topper-Wire-Harness-4-Prong.html

Buy this harness, connect it to your topper fuse box, then tap into the purple/gray wire going to pin 13 on the X1 connector in the junction box near the spare tire.

Connector/pin info: https://www.gm-trucks.com/forums/applications/core/interface/file/attachment.php?id=87676

Above info was for the third brake light. I imagine the 12v dome light will be the same process, just gotta figure out which wire you want to tap into.