Yup, I have this issue as well. It actually leaks on both sides. Have not found exactly where it leaks from. I would bet most, if not all XJ8/Rs leak water into the trunk; the owners just don't realize it because it's hidden by the carpeting. Who ever checks beneath all the carpeting?
Everyone should check under the carpeting and beneath the spare after a good rain or wash.
I discovered this when I removed everything out of the trunk to lighten the car for drag racing. I have not put back the carpeting since then, so the water can dry out when it does get wet.