From my understanding, water getting into the trunk under the rear windshield is due to the windshield seal leaking. Mine did this initially and I was able to renew the seal with an abundance of silicone lubricant (gel not the spray) using a plastic pry bar and an acid brush.

Many threads here, talk about having to replace the seal because it was severely damaged and cracking. Hopefully, this is not the case as most windshield companies will not guarantee against breakage when removing for replacement of the seal. If you have the laminated heating element, that windshield is ~$700-$800.

I haven't had the issue but I believe problems with the sunroof drains causes water entry around the hinge areas. Someone correct me on this, if needed.

Try the silicone lube first and see if it resolves the issue.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )