I found the problem, by using the flashlight from inside the trunk/ garden hose outside trick.
The water is coming in at the upper left corner of the back window glass. I will have to go to an auto glass replacement specialist and have them remove/re-install/re-seal the back glass I suppose.
Thanks to everyone for your responses.
BTW, I have already had the Explorer, I did like it, put 200k miles on her. Not ready to go back though.