4th Brake Light Help for 2017-2020 BRZ Stock Taillights
 

I need help with 4th brake light installation. I've got a 2018 BRZ with stock harness and taillights. I am also in the United States if that helps.

I decided to get this custom harness which has PARK, BRAKE, and GROUND to easily connect it:
https://www.facebook.com/marketplace...6890601436153/
https://i.imgur.com/VSXDoLT.jpg

I figured this harness would work as plug and play but am having trouble with it.
Also, I have this 4th brake light:
https://www.ftspeed.com/part/htyf86r...6#.Yfq_L-CIbqs
https://i.imgur.com/inqooxq.jpg

The website says OLM but the lens has 'Valenti' engraved.

Anyways, I unpinned the light blue wire from the black/light blue/green harness.
https://i.imgur.com/ljhUnY6.jpg
I connected the black/ green harness to the reverse lights and reverse works.

I connected the custom harness for the taillights and they are plug and play. However, I ran into problems when I plugged using the spade connectors.

I connected the red wire to PARK connector.
I then connected the light blue wire to BRAKE connector.
I left ground empty.
https://i.imgur.com/35zgHHS.jpg

When car is turned on, and lights are on, the taillights brake lights are on the whole time and the 4th brake light triangle flashes 3 times then holds light. The V light bar doesnt turn on. Reverse lights turn on when going in reverse.

I also used this diagram and confirmed that it is correct with my taillights. I also confirmed that the custom harness is correct using the diagram.
https://i.imgur.com/iImYdEF.jpg
https://i.imgur.com/KivRpAN.jpg

I tried connecting red wire to BRAKE and light blue wire to PARK. Got the same result. Any thoughts on this?

lights, cars, and people. all things that left ungrounded, have proven to have poor results.

guess you weren't looking at the custom harness

I can't easily understand the custom harness from the 2 pictures.

But lights need a ground/negative to work. It's getting it from somewhere. likely where your problem is.

I would start by tapping the wires of the 4th brake light to a 12 v source on a workbench and figuring out the exact combo for each function first. There's going to be a common ground and the other 3 will be functions.

But trying to deal with figuring the 4th brake wiring while dealing with the car wiring, and throwing in an adapter harness is way too many steps all at once for me.

Do you know the 4th brake light is good? Could be defective out there of the box...