Seal it from the top. It's right where the trunk lid hinge attaches. This is a very common problem with this car. I sealed them years ago with clear silicone. There are probably better sealers to use.
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