When I hook up my boat trailer and use the right turn signal in my 2002 Supercrew, both left and right turn signals activate at the same time on the trailer. Same thing happens when I use the left signal in the truck. I know it's not the trailer, as I hooked up a buddys and it does the same thing.
Can anybody help? Is it a relay, or would changing the wiring harness assembly make a difference?

 

If you have factory wiring it should be standard and work with almost any trailer wired to the same standard.
If the truck is a home wired deal then you just trace it with a test light and make the changes to match.
It seems the left and right turn sigals are crossed on the truck for some reason.
They must be seperate from the front, left and right circuits.
Just set one side flashing and do some testing on the oppisite wiring until you come to a point where there is a cross..
The problem usually would not involve fusing or grounds, only a cross /improper wiring.