[Ewr101]

Do you have a wiring schematic of the truck? That would help you a ton but if not, I would follow the wires from each brake light back looking for bare wire, bad splices ect ect.... If it's a single wire for each side and the tail lights work, then the problem is before they're combined into that single wire for each side. Also check to see if you're putting in the right size fuse. Simple things that I don't think of are what always gets me...