If you open the trunk and look on either side, there is a seam, outboard of the weather stripping. That seam was leaking on my car. I used clear silicone to seal it, and my trunk has been dry ever since.
_________________
98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54
https://www.youtube.com/watch?v=UHCda-t_Jls
https://www.youtube.com/watch?v=sfT2tEO4XcU