[bamashooter]

I'm assuming it's not your heater core since you didn't mention the smell of antifreeze. Possibly the condensation drain tube from your a/c. Another possibility might be underneath the fresh air inlet (grill in front of the windshield) might have it's drain holes plugged with grime allowing moisture to enter the cab. Can you tell where it's coming from as far as how high up on the firewall, kick panels, etc?