I'm writing this thread as a resource for those wanting to wire up a third brake light on the latest gen Ram 1500. This applies to any accessory install that requires wiring a brake light.
Other threads I've referenced that might also be helpful:
forum.gofastcampers.com
hdrams.com
Background:
I got a RSI Smartcap Evo Sport for my new 2025 Ran 1500 Rebel. For older Ram trucks, RSI provides a T-connector that uses your 7 pin trailer connector to power the 3rd brake light on the cap, or Center High Mounted Stop Lamp (CHMSL). However, the 2025 model (and maybe others) has sensors that can detect current flow through the trailer light pins. This is a novel feature, but it also means using these pins for accessory lights tricks the truck into thinking a trailer is attached, causing many issues.
RSI's solution is a different T-harness that is installed under the steering wheel behind the plastic trim. This requires you to run the provided wire all along the length of the truck, and to temporarily remove plastic trim to complete the install. This is a lot of work and not very desirable.
As noted in many threads, Ram trucks feature a spare CHMSL wire near the driver rear wheel. This wire can power any LED brake light (within reason), and is not used for anything by default. It is very hard to find if you don't know what to look for, which is why I'm writing this thread. But once found, it provides an easier way to power the CHMSL on your truck cap.
Wire Location and Info:
The wire is located between the driver side rear tire, and spare tire, taped to a thick wire harness up above the main bed support rail under the truck. The following video shows where to find it:
The wire is stranded and around 18-20 gauge, and is white with a purple stripe. This wire will likely be fully covered in electrical tape! As seen in the attached picture, the wire is just a bump on a large wire harness. For me, the very end of the tape was loose (maybe 1/2" exposed), which I could grab to unwrap it, revealing the white an purple wire (about a foot long ish). I used an 18-22 gauge crimp butt splice to connect this wire to the wire leading to the truck cap, and then added heat shrink tubing and some e-tape for good measure.
This wire gets 12V whenever the brakes are applied.
Grounding:
The easiest way I found to get GND to complete the circuit is using the GND screw on that same metal beam. On the same metal beam shown in the video, but on the opposite side of the beam (facing the spare tire) the wire harness is grounded with a ground screw terminal. I used a 5/16" ring terminal as shown in the picture and it worked well, though it was slightly big compared to the post. This config keeps the CHMSL and GND wires very close, making the install tidy.
Install Summary:
I fed the RSI-provided wire down through a hole in the truck bed, and cut it to length and stripped it underneath the truck. I used a heat shrink / crimp wire butt splice (18-22 gauge) to connect the white and purple CHMSL wire to the positive wire. I crimped on a 5/16" ring terminal to the ground wire, and attached it to the nearby ground terminal. I covered the wires in a corrugated plastic wire cover, and used heat shrink tubes, electrical tape, and zip ties to make everything tidy and robust along the way.
Even with all the threads I linked, it still took a few hours to find this well-hidden wire. I saw rumors that this wire was removed on newer trucks, and I can't speak for all models, but I found it on the 2025 1500 rebel so there's a good chance it's still there. Leave a comment if you have questions or want more info, I'll try to keep an eye on this thread and respond when I can. I hope this helps!
Other threads I've referenced that might also be helpful:
RAM brake light wiring
Pretty self explanatory but for anyone curious the red wire goes to red and black goes to either of the two blacks. On with brakes only.
forum.gofastcampers.com
Topper, camper shell wiring
I had a topper/camper shell installed on my truck. They hooked the interior light in the topper to the white wire with the tan/brown stripe on the truck. This wire is one of the three wires located above the spare tire for a topper/camper shell. It worked for a while, but now there is no power...
hdrams.com
Background:
I got a RSI Smartcap Evo Sport for my new 2025 Ran 1500 Rebel. For older Ram trucks, RSI provides a T-connector that uses your 7 pin trailer connector to power the 3rd brake light on the cap, or Center High Mounted Stop Lamp (CHMSL). However, the 2025 model (and maybe others) has sensors that can detect current flow through the trailer light pins. This is a novel feature, but it also means using these pins for accessory lights tricks the truck into thinking a trailer is attached, causing many issues.
RSI's solution is a different T-harness that is installed under the steering wheel behind the plastic trim. This requires you to run the provided wire all along the length of the truck, and to temporarily remove plastic trim to complete the install. This is a lot of work and not very desirable.
As noted in many threads, Ram trucks feature a spare CHMSL wire near the driver rear wheel. This wire can power any LED brake light (within reason), and is not used for anything by default. It is very hard to find if you don't know what to look for, which is why I'm writing this thread. But once found, it provides an easier way to power the CHMSL on your truck cap.
Wire Location and Info:
The wire is located between the driver side rear tire, and spare tire, taped to a thick wire harness up above the main bed support rail under the truck. The following video shows where to find it:
This wire gets 12V whenever the brakes are applied.
Grounding:
The easiest way I found to get GND to complete the circuit is using the GND screw on that same metal beam. On the same metal beam shown in the video, but on the opposite side of the beam (facing the spare tire) the wire harness is grounded with a ground screw terminal. I used a 5/16" ring terminal as shown in the picture and it worked well, though it was slightly big compared to the post. This config keeps the CHMSL and GND wires very close, making the install tidy.
Install Summary:
I fed the RSI-provided wire down through a hole in the truck bed, and cut it to length and stripped it underneath the truck. I used a heat shrink / crimp wire butt splice (18-22 gauge) to connect the white and purple CHMSL wire to the positive wire. I crimped on a 5/16" ring terminal to the ground wire, and attached it to the nearby ground terminal. I covered the wires in a corrugated plastic wire cover, and used heat shrink tubes, electrical tape, and zip ties to make everything tidy and robust along the way.
Even with all the threads I linked, it still took a few hours to find this well-hidden wire. I saw rumors that this wire was removed on newer trucks, and I can't speak for all models, but I found it on the 2025 1500 rebel so there's a good chance it's still there. Leave a comment if you have questions or want more info, I'll try to keep an eye on this thread and respond when I can. I hope this helps!
