Hello. I'm trying to find out what this part inside the trunk, under the floor panel is. I observed it started to get swollen.
Maybe someone can point me in the right direction, thank you!

 

You should have two pieces of styrofoam that fits to the right and to the left of that piece. The pieces that are not show will hold tools in the right piece and the one to the left holds the air pump and tire fix. That piece in the center of the car is insulation for the exhaust pipes which are directly under the car.

 

Oh I see now, so maybe it got swollen because of the heat. I thought it has something to do with the fuel tank.
There's nothing to worry about then, thank you so much for your help I really appreciate it.

 

it is insulation for the muffler so that the carpet does not get hot.

 

Side note; The pump is part of your security system and will go haywire if it gets wet.
Under that pump is a small black rubber plug. many of us have removed it to allow any water that may end up in that well to drain.

Also, since you don't have the Styrofoam box on the pump. you might consider putting it is a big zip lock bag.

I have replaced several of these pumps because of water. Since bagging them and pulling that plug no issues.

 

Thank you everyone!
That's not my picture, it's the only thing I could find to show you what I'm talking about. I didn't want to take everything apart.