http://www.championtrailers.com/Elec...l%20wiring.pdf
The cold wire is the wire on the brake light switch that becomes hot when the brake is pressed. The switch is located on the brake pedal. According to the diagram in my Haynes manual, it is a white wire. You should test it to make sure, because I have found incorrect information in this manual. Just put a test light on it, when the pedal is pressed the light should come on.