Gasket/Seal/Trim loose behind rear seat

I have a gasket/seal/trim that has become loose awhile back and I simply pushed it back into place. This was about 3-4 months ago. Today I noticed the same gasket/seal became loose again. Curious what this is and what might be causing it to become loose.

Is it a gasket or seal? Or simply trim? See attached image.

Additional notes: I haven't seen any water inside or in that area, so I don't think this is a water seal.

 

Where is that? Not a good pic at all of location. Hard to tell where that is.

 

Directly behind the rear passenger headrest. I can get a better pic in the morning when the sun is out.

 

After pushing the rubber seal back into place, it fell down again about a month later. Here are some additional photos. Any ideas what this rubber piece does/is for?

 

It looks like just some kind of regular bulb seal. Rubber gaskets like these wear out from exposure to the sun and heat as well as friction. It's probably just worn out and tired.

My dad had this Volvo growing up that had that happening to all the seals on the doors. Huge seals that dangled down and smacked you in the head when you went to get in the car.

I mean to really figure out what's goin on with this looks like you'll have to take out the headliner. Or glue it back up in there? Or just live with poking it back up there every 3 months or so.

 

What has its function?

 

Pretty sure that is just a trim piece to make it look purty.

 

Idk man it's probably some kind of bulb seal to weatherproof the rear window. Won't know for sure till you take that headliner off and get a better look.
If I were you I'd take a hose and spray down that rear window really good to see if it leaks. If it doesn't I'd just glue it up there and forget about it. If it does gotta pull that headliner and figure out how to reseal it.

 

I'm guessing its just to make things purty like @Jimmyh said. I live in southern LA and it rains down here pretty regularly. Major downpours at that. I probably would have seen a leak by now

 

Mine does the same thing. I'm pretty sure it's just a trim piece since it's on the inside of the truck. I haven't had any leaks either.