If your truck has a slider rear window, check to be sure drain holes are not stopped up. My trucks rear window leaked so, I replaced it with a non-slider glass. My leaks have been mostly the 3rd brake light, fixed with silicone sealer. I have also used silicone sealer under one of the roof black strips.
Those back strips just cover a body seam unless it’s rusted out that’s not your leak. Sadly the third brake leaks all the time the sunroof drains if you have one will plug and overflow also. Rear slider window has two drains that plug up also