Allpar.com has a solution, which refers to some holes or something under the body panel that fits between the trunk lid and the back window. This turned out to be not true for the Spirit I had.
Instead, the fault was the caulking, or whatever gunk they use, between the various sheet metal parts around the trunk opening. To describe where to look is a bit of a challenge:
Around the trunk opening, along the sides near the corners near the hinges, and between the flange supporting the weather stripping and the visible top of the rear fender, is like a channel to guide water around the trunk opening and off the back end. Near the corners, this channel has some welded seams. The body sealant used in those seams had dried out and was allowing water to seep through. The cracking was practically invisible, and I only tracked it down by gradually using newspaper to figure out where the water was entering. Sure enough, it was entering near the hinges, dripping onto the trunk floor and into the spare tire well. So you need to check these seams, dig them out and put in new sealant if needed.
I have not seen this described anywhere else and it took me a damn long time to figure it out. But it did stop the leaking on my car.
(If you live in a damp climate, there will be moisture condensing on the inside of the outer panel of the trunk lid. But the lid is designed to drain this water so it shouldn't end up in the trunk.)
If you have the folding back seats, moisture in the trunk will also cause annoying condensation on the inside of the windows.