I bought a 30 foot Coachman 5th wheel yesterday and am pulling it with my 06 Ram with a factory tow package. The 5th wheel has electric brakes and I'm using a Tekonsha P3 brake controller. When I plug in the 7 way I have turn signals and brake lights but when I turn on the headlights the running lights come on AND the brakes lock up. I've pulled off the plug end from the 5th wheel and checked the wiring. Red to the red post, black to the black post etc and I checked the junction box to make sure everything was wired the same there. The guy I bought it from said he never had any problems like this.

 

I'd guess regardless of wire color, the wires in the trailer plug are not configured the same as your truck. If you used the factory supplied pigtail to attach the brake controller to your truck the problem just about has to be at the trailer.

 

Sounds like the brake / running lights are reversed.. (does the trailer lights come on when you apply the trailer brake override switch?)

Not all 7 pole trailer plugs are wired the same get one of these start with the truck and work back:

http://www.delcity.net/store/7!way-tester/p_791230.a_1

 

Sounds Like The Blue Wire At The Trailer Is Attached To The Taillight Wiring.

 

Maybe a colorblind person wired the trailer? I have a hard time trusting wiring that may have been "worked on" by someone previously who may have neglected a certain convention with the color of the wire.

I'd go over the whole thing with a tester and some test leads to make sure each wire is doing what you think it should be doing.

 

never go by wire color!
going in a clock wise direction looking at the truck plug.
top right is power
right is right turn
bottom right brakes
bottom left gound
left left turn
top left tail marker
center backup lights

the trailer should be wired the same i think
your trailer plug in the bed is gooffed up
use a test light and make sure of each pins power orientation

 

yep ive been burned by color

 

Yep don't trust colors, my current trailor gave me nothing but fits untill I took the lights completely apart. Found one light had a black grnd the other light had a white grnd all internal.

 

Loss of ground on a tail and stop light will allow it to backfeed the brake controller when the running lights are activated.