I have a 2018 premier RS sedan, I had water coming down in that same spot and pooling in the spare tire well. It seems that dealerships are hit and miss with repairing these leaks, so I fixed it myself. Mine was leaking at the far left and right of the spoiler. There is a plastic pin on the spoiler that snaps down into the trunk lid on both sides. That is where mine was leaking. I removed the spoiler put sealant around those pins and all the studs, remounted the spoiler, and the leak is gone. I would keep a eye out for more water in your trunk, I had to remove the light housings out of the trunk lid to access the spoiler studs and nuts. I don't see how those light housing gaskets would just start leaking. Good luck!