I struggled to find a comprehensive guide for 22.5+ Silverado third brake light wiring, here’s what I learned through my research and install. I’m not an electrical engineer.
Issue
Newer Silverado tail lights use the same wire for brake signals and turn signals. In order to have a 3rd brake light that does not flash with the turn signal, you have to use a logic box (which separates the turn and brake signals) or splice into the CHMSL (Center High Mounted Stop Lamp) wire under the dash.
I struggled to identify a safe, dry place to secure a logic box and there were mixed reviews on the products, so I opted for the CHMSL solution.
Nearly comprehensive solution
This video demonstrates how to execute this project, but the BCM plug/pin and CHMSL wire color vary for each model year.
Identifying the CHMSL wire (the only hard part)
Go here and locate the “Electrical Body Builder Manual” for your model and year, then search for “Center High Mounted Stop Lamp”. You’re looking for the BCM pin out for the Lighting Control Module. I’ve included the screenshot for the 2024 Silverado 1500 below. Note that the “Connector Part Information” includes the color of the BCM plug. In my case the plug was blue and the CHMSL is identified as the brown/yellow wire in pin 25.
My install approach
I followed the install from the youtube video above with a few changes. I doubled up on protection for the wire I ran to the cab. I used a t-tap for splicing into the CHMSL wire instead of soldering. I removed the posi-tap connectors from the GFC pigtail and connected to the pigtail via butt connectors. At some point I’ll probably cut the extra harness out and install a new connector to clean up the final product, but it’s functional and protected. I chose to ground behind the left rear fender, which required removing some coating for electrical contact.
Materials
I’m a new member so I am limited to 2 hyperlinks per post. Once I shake this restriction I will add links for this list.
18awg wire (red)
18awg wire (black)
Braided sleeving (25ft)
Split wire conduit (25ft)
T taps
Butt connectors
Ground terminals
Inline fuse holder
10A fuse
Zip ties
Various heat shrink pieces
Necessary tools
Wire strippers
Crimpers
Heat gun (or a lighter)
4 beers if you’re slow like me
I hope this helps someone else out.
Cheers,
Tucker