I had that happen to my car. I removed the 1/8th inch thick OEM tail light gaskets and replace them with these 1/4" thick gaskets. They are sticky on one side. My trunk was filling with water. Now, it has been dry for years.
I usually like to make things myself, but do to the complexity of the gaskets shape and the (IMO) low cost; I just bought these off ebay.
https://www.ebay.com/itm/2013-2016-S...UAAOSw5dlaA0dm