Not sure if you ever got an answer. Had similar problem. Basically crap had gotten into the drain tubes over time and plugged them so water was backing up and running into the interior behind the headliner and down to the rear floor boards. I used an air compressor to blow out the tubes from the top. Also ran hot water with disinfectant cleaner down the tubes and continued to flush with hot water. If you pour water into the sunroof rear channels and don't see water running out behind the rear wheels, then the drains are plugged. Try and get some good pressure through them to clear them out.