I put the screw back in, coating the threads with the 3M Window Weld Urethane sealant I bought to reseal around the front and rear glass which I first suspected.

I had also had a leak on the back glass. Again, at first I thought it was just a poor job by the glass shop on the installation. Turns out that last year while doing body work when I tried to repair some pin holes (with torch and brass rod) in the lower right corner of the channel that I inadvertently melted out the body joint sealant right where the quarter panel and the metal panel behind the rear glass meet. I spent the last two days feeding drops of silver POR through the passage until I was sure it was solidly filled.

Today I performed two water tests and am glad to report I see no water leaks!!

BTW, my own idea on preserving the lower corners of the rear glass channel is to completely fill the corners with the urethane window sealant. If you ever have the window trim off I would recommend thinking about doing this before its too late and your corners are rusted through.