It's not me....
I was worried that the blame would eventually get back to me!
I don't think it is the recirculate mode since I rarely use it. And to answer Tobias' question, it's an 89 260E, but the carpets are very dry with no sign of leakage. (Although the driver's side carpets and mats were pretty dirty. And the fact that this is a black/black car is not making it easy to visually determine how clean things are.)
However, since you mention it, the one area of the car that does gets wet is the left-side well in the trunk. Two years ago, that used to get quite flooded in the rain but now, at best, it just gets damp.
I'm focusing on that area and the HVAC and hope I'll find the cause eventually. (I've been on a winning streak fixing this past week's door lock, alarm disconnection, ignition disabling, and tranny problems, so maybe I'll finally conquer this one as well.) And if/when I do find a solution, I will definitely post it here.