Water leak in back seat and back of seat

I have water that wets the seat and the back of seat on rear passenger side when rains hard. Doesn’t get wet behind seat in storage area though. Anyone have idea where comes from?

 

Most common - 3rd brake light. There were TSBs about it. My thoughts - roof rack channel or window seal.

Pull trim panels, grab a buddy and a hose, and chase it.

 

It's likely the third brake light cuz it's in the back, but it depends on what direction your truck is parked, uphill or downhill. I recently had leak from the sunroof that ended up in my front passenger side footwell. Pro tip, the weed wacker line that I have, Echo with a twist, was perfect for pulling out debris from the sunroof drain.

The drain slots are in the front corners of the skylight ( assuming you have one). Pour a cup of water in the corner to see if the water drains down and out, and you'll eventually hear a drip underneath the truck. If not, then the drain is likely clogged, and rain will eventually make it into the Skylight and down your headliner.