Leaks like this can be hard to track down. Since the trunk is big have someone get in there with a flashlite and carefully pour water over key locations one at a time. Places like trunk seam, rear window and at the missing gasket location.
I had a leak in the Z-28 long ago that got water under the rear seat and traced ot to the left dor=or where window glass met body so it's not always a direct patch.
