Water leaking through this part

[YKULKAR]

Hello everyone!

I have 2003 Corolla LE model. This morning I noticed water dripping from the top side of this part (circled in red)


When I inspected more, I found that the part was kind of loose wrt to its counterpart at the driver's end. So I applied gentle pressure and pulled it out.. There was some water deposit (slight) in one of its pockets, I emptied it and now, I am unable to put it back on..

Could someone tell me where the leak could be coming from? Also, what is that part called and how I can put it back in? I could find nothing on Google.

Thank you.