I Need help from Leer cap owners

I have a 2011 Taco. Just recently got a 2009 Leer XR shell and love it. I wired it up to my brake light. I just noticed that the 3rd brake light works fine when driving regular but when i have on my head lights my brake lights still work but the 3rd light does not? Turn off the head lights and 3rd light works fine. Fuse is fine, no short in wiring. I wired it to the two wired directly from the brake light. Any Idea's??? THANKS!!!

 

Maybe you have them reversed?

 

Which reversed? The + and -? I'm willing to try. Just does not make sense to me of why only when head lights are on. No other problems with it though..

 

I figured it out!!! The Black wire must go underneath your truck and be grounded to the frame of the truck. A Leer distributer said Tacoma's are known for the bed of the truck not having a good ground so run it to the frame and PRESTO! Works like a charm.