Is it wet with coolant or just rain water?  If it is coolant then you either
have a hose or heater core problem.  If it rain water then it is probably the
gasket for either the windshield or your radio antenna.  The gasket for the
antenna would be the first one I would check as if this is bad, rain will get
in and run down your antenna wire and drip behind the dash at its lowest
point.  Hope this helps.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242