THIS! This is my exact problem! It’s that freaking tail light and leaking when I open the trunk! Water pours down that arm thingy. Thank you for your post, now I know how to fix mine.
I too had some water leaking into my trunk from inside the trunk lid liner (that carpeted cover on the inside of your trunk lid). I had my local KIA dealer look at it and they discovered the backside of the left tail lamp assembly (the one fitted to the trunk lid, NOT the car body) had a hairline crack in it and was allowing water to enter & pool up in that tail lamp. When the trunk lid was lifted open, the water would then drain out of tail lamp assembly through the bulb socket, run down the trunk lid liner and exit through a small opening next to the left trunk lid arm, dripping water inside my trunk.
The dealer replaced my driver's side trunk mounted tail lamp assembly for FREE under warranty, which fixed my problem; no more water leakage.
So, if you have water leaking into your trunk, CHECK YOUR TAIL LIGHTS for condensation or water inside them. If so, that may very well be the source of your problem.
I hope this helps someone. Good luck.