I have had the same issue. I have a 2017 Mustang GT Premium. Triple Yellow with all blacked out. It was on the showroom floor inside when I bought it and garaged for about a year when not driving. I then had to park it outside a few months ago and when my daughter got in after a rain the passenger rear seat belt was soaked. You could pull the whole belt out and soaked with water. The seat was wet as well. It leaked the same spot you described. The water also went into the spare tire area and would fill with water. I took it to dealership and couldn't get it to leak. Went home and leaked again. Took back and still couldn't find leak after testing. Went home leaked again. This time I got it on video. So they thought it was a seal above door and tried to fix it by replacing that. Didn't work. Leaked again and brought in 30 minutes after I picked it up. Now I called the Ford people and car back in shop for almost 2 weeks and still cant get it to leak. They see water spots inside where water was. Now the Ford Rep at headquarters wont let me have it back yet because I think they almost at the buy back stage. Also both puddle lamps have been replaced 3 times. Love the car but this is crazy.