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Have you tried signals? If they work and your lights do not then your trailer ground is good and you might have the blown fuse that I had.
When I first got my '08 I tried to plug the old-style trailer plug from my concrete pump into the plug on the truck upside down. I think it is opposite the plugs on the earlier models. Anyway, this caused the 30AMP maxifuse for the trailer lights to blow. It is located in the fuse box under the hood.
Just a guess. Good luck.
Trailer lights fuse is under the hood? No wonder I couldn't finsd any blown fuses under my dash or anything coded for trailer lights, only brake lights. I will check my under hood fuses tonite. Brakes and turn signals work good, just no running lights on the trailer. I check the 7 pin plug and the 4 pin plug too, no 12v. for running lights. Must be that 30a fuse you mention. Thanks!