I’m sharing this a few places, hopefully it’ll help future GFC Rivian owners.
Wiring the rear brake light:
Step one
Remove the rear seat bench
it is held in with two 15mm bolts under the headrests, on bolt on the bottom middle (15mm ) and one nut centered under each passenger back area (15mm as well)
don’t forget to unplug the two white plugs behind the seats.
Once removed, pull the small kick/ sill plate from the driver side floor
peel up the next two trim pieces, the dark grey going up the side and the lower headliner piece that matches your interior and has the seat belt passing through
In the loom locate the plain green and plain black wire (black wire is smaller 22awg)
verify it is the plain green and not the stopped green
this is the chsml pair
I tapped into the wires using a positap
Highly recommend
You can get them at O’Reilly or Amazon
In the truck bed remove the rear plastic panel it is held in with 8 M6 (10mm socket) bolts that are hidden under the plastic circles
once pulled you can see the tonneau stack if you have it and a few grommets
I don’t have a tonneau so I used the higher location to not have to fiddle with the sliding over the gear tunnel
you can use the lower grommet and fish any wire around the felt of the gear tunnel and out the space behind the passenger seat
Drill a hole in the plastic piece in the location of you liking, or figure out a seam to tuck the wires through if you are nervous about drilling
I don’t like pinching wires so I went the hole drilling route and will use a weatherproof mini box as I have other cables using this area to run power
hook up the lights to the provided pigtail (gfc came with some positaps, which I moved to the splice location, and then used the twist connectors to connect the short length of jump wire needed
that’s it!
It works
No extra sensors or modules to burn out
No false trigger of trailer mode
No issues with turn signal behavior
it just works.