I have a 2001 Expediton. When I run the A/C, water leaks onto the passenger side floor. If I have a clogged external drain, how can I locate it? I have already looked around, but nothing is jumping out as a possible drain. Thanks, Drew

 

Most probably the heater core...........

 

The drain is located on the lower right side of the engine compartment firewall, behind the engine.
As another poster said, it could also be the heater core. Check your coolant level. If it's low, it probably is that. You can also tell by the color and smell of the liquid on the floor.
edit: hehe, just noticed the date of the original post. Didn't mean to bump this outdated post, sorry. =p

 

it could be the heater core.. Indicators of a leaking heater core are the same symptoms that you have come to expect — a sweet smell in the interior with the defroster fan on, an oily film on the windshield, perhaps a leak out of the evaporator condensate drain or a slight mystery coolant loss. Always inspect the evaporator condensate drain located above the transmission bell housing.

 

Yea somehow they sneak up on you sometimes