Well, I tried checking again and there are no leaks from the places I'd already checked.

This is really baffling!!!

 

OK, I'm back. Knock on wood, it's fixed. Here is the story. I took it to the shop that installed it and has been investigating. They took apart the installation. Both drains were clear so that wasn't the problem. Then, instead of pouring water into the channel overall, we poured just a little in specific spots in the channel. Nothing leaked util we found one spot directly over the place where the water was leaking in down the left interior panel. The shop tried to put a patch over just that area, but it didn't work. Finally, they installed a vinyl film along the entire channel and that stopped the leak. I can't envision this, but he told me a bolt that connects the top to the body caused the water to dam up and overflow into the trunk.

I hope I'm describing this properly because I'm not completely sure of the terminology. I think several people on this thread were in the right ballpark with their input and thank them.

The owner of the shop said this was the trickiest problem he has come upon in his 40 years in business.