Have an 89 Caprice but I get a lot of water in the trunk after a rain shower. The truck seal is like new so I thought it was the rear window and took it to a glass shop. They said no as there is no sign of water on the package tray. Their thought was a body seal was old and cracked. The suggested I get some "seam seal" and try to re-seal it from inside the truck. It is difficult for me to crawl into the trunk and fish around for seams so can anyone offer other suggestions? For the moment I have covered the two seams between the panel under the window and the fenders with duct tape and will try to run some water over them.
The car is very low mileage and has sat for many months with very little use.