I'm not quite sure where it's coming from but it sure is coming. The photo below illustrates the obvious contrast between the dry area of my headliner and the VERY wet area.
I'm fairly certain, based entirely upon what I know of water and the effects of gravity, that the leak is somewhere above the top of the windshield. As luck would have it, there is only a relatively small percentage of truck above the top of the windshield. This should make finding the source of the water somewhat easy.
I first inspected the rubber trim/molding covering the seam where the windshield meets the roof panel. I found no cracks, tears, bubbles, gaps or otherwise obvious places for water to enter.
I opened the driver's side door and inspected the end of that same rubber trim/molding. there appears to be a rather conspicuous air space where this trim/molding meets the rubber door seal and the vertical pillar trim. The space is not however excessive and appears to have been installed that way right from the factory. GM would surely have tested this configuration and ensured that water would only enter under the most extreme conditions, right?
Then I went over the passenger's side to inspect the other end. It seems the rubber door seal has shrunken substantially and opened that air space on this side much larger than on the driver's side.
This I suspect is the source of my leak. As the seam where all four doors meet the roof provides a well defined channel for nearly every drop of rain to run down into the HUGE opening where these plastic/rubber seals/gaskets/trim/moldings converge, I'm rather surprised there isn't more water inside. (note: I soaked up the small puddle in the console tray with a paper towel before it occurred to me to take a few pics.)
Now the question is, how do I fix this? My first thought was to stretch the rubber door seal back into position and hold in place with some silicone caulk. That's probably not the best fix as it will likely keep the temperature sensitive material from expanding and contracting freely and likely shorten its life and cause far more serious leaking problems in the not so distant future.
What's holding that rubber door seal in? Is it just molded rubber squeezed into a channel? That I could pull out a portion of and reposition?
In the mean time, I have solved the problem temporarily by parking facing up hill so that any rain or other water that falls upon the roof runs off the back instead of down the canal to the headliner.
