I have the same problem. The seal surface and out toward the "skin" is rusted thru. Looked difficult for an amature bodyman like myself. I'm spending $$$ on a decent paint job, and want to keep water out of the trunk, so I bought a new AMD trunk lid. I'll supply photos and the good and bad when both rear quarters are replaced and all parts fitted to each other. The deeper I get into the body, below all the old paint and bondo, the more trouble (spell $$$) I'm finding.
Good luck.