I have not had this issue so cannot provide an answer to you. When I clean my car, in or out, I use a flat screwdriver wrapped in a terry cloth and run it in the weather channel from the rear glass to the front of the rear roof section while roof is in service position. The weatherstrip channel is pictured in your photo. This always gives up some detritus, pine needles, and decomposed plant material. I try to check drain at the same time and have not had to ream it as you seem to require. I have parked outside until recently where I now have a garage so will likely need to do this less often. My Miata had overflowing drain in right passenger area that soaked the carpet and a coat hanger was the occasional answer for that. It was a straight shot to the pavement from the hidden access hole, once I found it. This is what led me to be active with my C70 so the same would not happen (though it still could).