I have a 2003 F-150 4x4 with the standard towing package. I want to hook my brake controller into my reverse lights so that it will cut-out when in reverse. the factory wiring harness does not have this wire included in it. so my question is, where could I fing this wire at (preferably in the cab) so that I can tie into it?? I pulled the tail lights off and found that the wire for the reverse lights is black W/purple stripe, but i wasn't able to trace it anywhere.

 

I have to ask, why do you want to do this?

And if you find the wire, you will probably have to add a relay correct? You are going to be breaking the brake signal wire when you get voltage from the reverse circuit.

On the older trucks, the reverse switch was on the side of the tranny, and the harness usually ran up the driver's side firewall in the engine compartment.

 

well the reason I want to do this is because the brakes on my trailer seem to mess-up when used in reverse. my controler has a wire that is made to hook into the reverse lights. when this wire receives voltage(when truck is in reverse) it cuts the brake signal out. however you can still manually aplly the brakes with the controller box.
I tested this with a jumper wire today.

 

That's a pretty neat feature. I wish I could help you with the back-up wire, but your truck is newer, and I don't know if they still put the switch on the tranny, or if it's on the shifter linkage or column somewhere.

You could always put a manual switch under the dash somewhere, but there is always the risk that you would forget and leave it on and then not have any trailer brakes.