its not hard to do, just time consuming because you need to remove a lot of stuff.
Visors, overhead console if equipped, grab handles/pillar post trim, upper seat belt anchors (so you can remove the pillar post trim covers), coat hangers, rear dome light.
Even after all of that, I could only get the headliner down just enough to get my hand in between it and the airbags (if equipped)....in my case I did it so I could make sure the shark fin antenna was sealed to the roof (was getting some condensation inside the truck during certain weather and wasnt sure if leak or just hot-to-cold changes).
FWIW your leak looks *exactly* like mine, which was the third brake light. One damp corner of headliner and a few drops of water on the black bezel around the rear glass. Mine didnt make it down to fill the underseat area (AFAIK), but once the dealer dealt with the third brake light, it didnt leak again.
