I have a 2004 GMC Sierra 2500HD and ever since ive had the truck ive had trailer light problems. The problem I have is I dont have running lights and the right side brake and turn signal do not work. I have replaced the fuses for this but the second I replace the fuse and turn on the turn signal or hit the brake it blows the fuse and If i flip the 7 pin connector around I wont have running lights or the left side turn signal or brake but will have the right side. If i replace the fuse and check the voltages to the pins without a trailer on everything is there correctly and will not blow the fuse the secound I hook the trailer up and turn on the turn signal or hit the brake it blows the fuse I know its not the trailer because It works fine hooked to other trucks. Been trying to figure this out now forever please help!

It seems you have electrical wiring problems. if the fuse continues to blow even though all the connections are properly connected, it simply means that there is a 'live' wire that is being touched by a 'ground' wire.

live wire is the wire connected to the positive(+) terminal of your battery. ground wire is the one connected to the negative(-) terminal. the whole body of your truck is connected to the negative terminal, thus the term body ground.

have an electrician trace your wiring connection.

some common causes of faulty wiring are:
1. extreme heat that can melt the insulators of your wiring
2. water that comes in contact with your wiring
3. rats/rodents that eats everything including wirings =)

Check the plug wiring again on the truck. You have a wire crossed , when you hit the lights and it only blows with the trailer, then there is a crossed wire on the truck. What seems to be happening is your sending power to the ground for the trailer. You can check the trailers ground with an ohm meter, then make sure your truck ground matches that pin. You may know this , but green rght turn, yellow is left turn, brown is parking lights, black is brake lights, and white is ground.