there are fuses for the trailer under the hood and in the fuse compartment on the driverside dash, I would check both, is there another truck to plug the trailer into to see if it works correctly. Also just as a shot check the bulb in the trailer the bulb should have 2 filaments, one for turn the other for stop, one filament could be burnt out and the other still good, worth a shot.
7 pin was factory installed. I will check both fuse compartments again, but I'm almost 100% sure they were all good. I will let you know what I come up with.
I have an 04 pickup with exactly the same problem, I hope someone can help. Diferent trailers do the same thing.