After resealing the window, the windscreen, the side mirror, the snap-screws for the windshield shade, etc. etc. the pesky leak was still not resolved. I went back up on the roof this morning (after applying more DICOR to just about everything a couple weeks ago) and took another look around. And... I found a crack in the Dicor on the front mount for the air horn assembly. This crack was not visible before, or maybe I missed it, but in any event I am hopeful I finally found the source of the leak. See photo below.
I scraped away some of the old Dicor in that area and reapplied VERY liberally. Now waiting for another heavy rain to verify - shouldn't be long, this time of year in Livingston TX is rain, rain, rain!