2002 2500 with I think factory tow package has both 4 pole and 7 hooks up. The 4 way works like it should.  Turn signals and brakes. The 7 way only had turn signals and brake light brake box wouldn't come on either.  Checked the 40 amp fuse under the hood for the trailer and it was bad swapped it out. Brake box worked and had running lights. Then the ground wire on the trailer was smoking and my third brake like was stuck on till we unhooked trailer. What is wired wrong on this truck? It ain't the trailer.  I think something was odd about dodge wiring but I cant recall what. Any help is appreciated. 

Wireing is weak on these, wires get hot and melt especially with bad grounds. Check brake switch on brake pedal and light switch on dash, and grounds associated with it.

I haven't seen both 4 pole and 7 pole set up from factory, just one or the other. If its a factory 7 pole, maybe someone spliced a 4 pole plug into it, I would resort back to all the factory wiring and check everything over, then if a 4 pole is needed just purchase a 7 pole plug with a 4 pole adapter.  note that wire colors can differ from manufacture put the positions should be the same

My 02 came stock with both. Part of the camper package I am thinking.

It seems to be wired up right from a test light but something is wrong somewhere.  It could be aftermarket but it looks really close to a factory look. 

hmm.    well like I say I haven't seen one, but it would make sense if you had to plug in a camper and tow a trailer at the same time.  but for the OP it sounds like got some wires crossed somewhere or a short to ground. maybe try checking each circuit separately with a jumper wire, between truck and trailer,

 

 

 

Now that's an idea! I dont know why I didn't think of that thank you! 

2 hours ago, dripley said:

My 02 came stock with both. Part of the camper package I am thinking.

 

My 02 also came with both.  I pull three different trailers with trailer brakes using the 7 prong terminal.  Been doing it for almost 290,000 miles and I have never had a problem with the trailer wiring circuit.

 

Unreal Summit, if you are not the original owner of the truck and you noted that the 40 amp fuse for the trailer circuit was blown, then it is likely that a previous owner did something, (I am sure unintentionally), to this circuit.  01cummins4eve is steering you in the right direction.

 

- John

19 hours ago, dripley said:

My 02 came stock with both. Part of the camper package I am thinking.

 

Both my 97 and 01 only had the 7 pole. Not saying it is not possible they changed in for 02.

I did have a 7 pin added in the bed when new and unless they added a 4 pin at the stock 7 pin location location it came on it. I sure did not add it myself.

 

I do use both of them, one for the 5th wheel and the other for smaller things from the rental yard.