So I bought one of those slim-line LED brake lights that go between the tailgate and bumper. I've got it all mounted up but now I need to wire it in.

It came with a flat-4 prong adapter to plug it into my trailer wiring, but I don't want to have the connection visible. Not only that, but I want it ONLY for a brake like - I do NOT want it on with the tail lights, and I do NOT want it on with either turn signal.

I ran the wire up behind my tail light trying to tap there but I'm not having any luck. Tried shooting voltage with my multimeter and no luck.

What wire can I tap into to give a signal to it for the brake lights? Also, will I need to ground the wire that normally grounded through the trailer connector?

 

Wire it into the third break light