Well, whatever it was has gone away: exactly the same climatic conditions, put it to bed after driving home last night after the Blazer game with the climate control on ECON @ 70 degrees and the interior glass is as dry as a bone today!
Think I'm going to clean out that drain line anyway.
Thanks to all for your suggestions, although some of them seemed to indicate a much bigger problem than I had ever anticipated. At least now I'll know what to look (OUT!) for if I ever see a lot of moisture in the interior during the winter.
BurgandyBeast: I sure hope your core is not the problem! It sounds like you have much greater symptoms than I had.
Thanks again!
STORMIN'