Haven’t seen this posted anywhere yet personally and see a lot of folks ask.
How to wire the third brake light on the GFC.
(DISCLAIMER!) This is for my 2017 3rd Gen Tacoma. Your Tacoma might be different. If you don’t have a Tacoma it will positively, probably be different.
(1) I started by removing my rear, driver side taillight.
(Two 10mm Bolts) Circled in photos.
(2) I then fished the wiring through the gap in the bedside.
Be patient doing this because you don’t want to risk damaging your wires.
It might help to pry the metal/plastic out a bit with a flathead screwdriver inside the bed.
If you’re lucky you’ll be able to catch them on the inside of the hole where the tail light is. If not you’ll have to reach up from the bottom and push the wires through or remove your drivers side bed box cubby if you have one.
(3) Now that you have your wires where the taillight goes, look for the middle (egg shaped) connector. This is the connector you’re going to need to tap the two wires into.
Remove and skin back the tape on these wires a few inches.
(4) Get a idea of how the needle wire splices (that GFC provide) work before attempting to use them.
YOU DO NOT NEED TO CUT YOUR FACTORY WIRING!
YOU DO NOT NEED TO CUT YOUR FACTORY WIRING!
Both the black and red ends on the splices are removable. Take those all apart!
The red end on the wiring will act as a swivel when you’re putting them all back together so you don’t have to twist the wiring.
(5) Light Blue = Positive +
White/Brown Stripe = Negitive -
(6) Confirm the plugs Black/Red orientation is correct inside the bed. Make your connection…
(7) Replace taillight. Don’t pinch wires.
(8) Test it.
The light only comes on during braking wired in this orientation. It will not function as a DRL.
It looks like yours has a hole on the side that mine doesn’t? Where I drew the x. Some vids online show it going through the small gap in the floor of the bed but that seems too small. Any clarification would be so appreciated! I have a 2020 Tacoma DCSB. 
