2015 crewmax water found in rear floor board

We have had a lot of rain here in the past few weeks. I got in the truck the other day and it smelled just terrible. I lifted the rear seat up and discovered the rear seat “legs” had some surface rust as if they had been wet before. Water had also puddled by the door plates. I immediately thought it was the 3rd brake light but there is no evidence of a leak in the rear headliner. Is it still the 3rd brake light? Or is there another problem area I should check?

Thanks for the help in advance!

 

Could check the door seals, possibly sunroof, or have you left the rear window down any, I know i have on several occasions.

 

Possibly the door seals. I don’t have a sunroof. But it seems to far back in the floor board to be the door seal.

 

I would pull that lining on the back wall behind the seats and see what's behind it. Can you get in a dark area and pull the headliner to see if there is any light around the 3rd brake light? I thought someone on here had theirs leak but no evidence of it on the headliner. I would also feel the carpet where it meets the back wall.