You might consider looking into the possibility of leaking around some breach in the firewall. Does your car have the "sound insulation panel" (what I call the belly pan) underneath the engine/transmission? If so, it should keep out almost all environmental water intrusion from below.

You say the water appears clean - does it have a smell? Does it have any color? I was thinking that it might be something that runs in from the engine compartment along a hose or wire(s) that run into the cabin. I was thinking windshield leak (rusted out area underneath the rubber molding that holds the window in) but you said that the car was dry after a hard rainstorm...

Good luck - since I have the same year/model car I will be watching this. Once you've figured it out I'd like to see what you found out.

__________________
Jay Yambrovich
Scotts Valley, CA.

1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles

2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)