What is likely happening is water is riding the channel of the trunk opening down to the panel, running between the gap of the body and GV panel. Where it then drops into the opening of the adapter plate that the taillights slip through. That's most likely where the water is getting into the trunk.I got mine fitted pretty much the way they'll be, and they leak water into the trunk like a bitch. The design doesn't seem to seal very well. Any tips?
So, how to solve it? Silicone between the body and panel where they meet. I got lucky being that my car is black and I was able to use black silicone for a near blind to the human eye look. I simply wore some rubber gloves (I recommend nitrile) and used my finger to smooth the silicone in the joint. Also used some paper towels to wipe away excess.
Here's what it looks like with camera flash.
And without camera flash.
As you can see, the only way it really stands out is with the flash of a camera. Clear silicone will like look fine with other colors. Or, and this is just an idea off the top of my head, but maybe it's possible to dye clear silicone with food or fabric dye.
If you were to try to make a gasket or use some silicone on the inside where the interior pieces of the kit mate to the body that wouldn't work because of the slots in them for the wiring. And if you did manage to seal that, water could build up in that pocket the taillight sits in. And I'm not sure how you'd seal the opening of the adapter plates against the taillights and still be able to remove them easily.
Anyways, after I did this, not a drop has leaked into my trunk.