what is the best way to identify the brake light wire on a 2003 tacoma? I need to connect to hook up 3rd brake light for truck cap.

 

Well, one way is to look up the wiring colors in a Factory Service Manual wiring diagram.
Another way is to pull the tail light assembly out and see which wires are hooked to the brake light bulb. Likely a dual filament bulb, so either trace the wiring in the socket or use a meter or test light to find the wire that gets power with the brakes applied.

 

thanks, couldnt see the forest thru the trees. when I connect the ground from the cap light, does it need to go to the ground wire for the brake light?

 

Probably would be a good idea, and you have to run the power wire anyway, so just as easy to run 2 wires vs. 1.

 

Probably. Much to my surprise, it appears that the bed on my 1994 pickup is insulated from the frame! (Probably for noise reasons, not electrical.) When I installed the in-cap lighting I had to pick up the ground from the taillight assembly.