What kind of car is it?

In my 1999 Alero, I noticed that the foam between the panel underneath
the glove box (which covers the blower) was saturated - but the only
way I noticed was when I was getting at the blower resistor pack to fix
the infamous "two lowest fan speeds not working" problem.  It never
leaked into the car.

I suspect that it was rain water from the heavy rains SoCal got last
winter, but I had no way of "testing" this theory, as it dried up by
the time I had done this work.  I'm not sure where the water would leak
from, but I guess it could be anywhere.

Sounds like a heater core.  Very typical in my experience with gm
products. Another option is your drain holes are plugged. I seem to
remember a recall on that product in the recall bulletin. You have to
enter your search criteria in google or some such
type "recall notices+olds or whatever your car is should give you the
search page on the bulletins.