I am about to go nuts. This is not my first time wiring a trailer. I have just wired my bosses trailer with a four prong harness. Everything is brand new, Wires, Harness, Bulbs, Clearance Lights and tail lights. All lights and turn signals, hazards, and brake lights work properly on the truck. Everything on the trailer works as well EXCEPT the brake lights they will not light up bright when brake is applied. I have checked fuses in truck and under hood and there all good. I have checked connections cleaned them and changed bulbs just to make sure. Still no brake lights for trailer. I have read some other forums and found that it may be the hazard switch, the truck is a 03 chevy 1500 and the brake and blinker lights on truck are separate. That switch is said to regulate between the two separate lights on the truck and when bad the truck lights will work properly but not for the trailer. Also i can only find threads where they refer to the 01 and 02 models doing this. So can anyone confirm the 03 will do the same if bad or am i missing something?

 

Test to see if you are getting power at the trailer plug. If you are then my bet is you have a weak ground. I have a 20 foot trailer and for some reason I had to add an additional ground location at the rear of the trailer (through the frame of the trailer). I was having the same issue and read somewhere that it could be ground related. Once I added the secondary ground it worked like a charm. I just ran an extra wire from the front ground location to the rear and then used an existing bolt that went through the frame to put a wire eyelet on. Good luck!