Where do I start?

I haven't seen any holes in the floor while looking under the car, though I guess to be thorough I'd need to pull the carpet up.

 

If it's the 'top hat' part of the cowl that's rotten or leaking, the water could be running right into the heater box and then dripping out anywhere along the seam, causing the floor to be wet, instead of the torque box area (the incline where you'd put your feet). Every case is a little different, but I would still strongly suspect the cowl.

If it was a heater core, you'd get steamy windows (most likely) and you would most likely be able to smell the coolant. See if you can notice any stains or signs of water running around the big, thick rubber gasket that sits between the cowl vent and your heater box (passenger side).

Good Luck, and hopefully your leak is found...

 

The cowl side drains get plugged up with leaves and pine needles and the cowl fails to drain - the water then builds up in the cowl and, since Ford never coated the inside of the cowl with anything, the hats rot or the bottom of the cowl rots through letting that water come into the car and then rot the front floors

Ask me how I know

 

When your cowl had to be replaced, what exactly was soaking on your floor? In my Comet it's specifically where the passenger would put their feet, minus that incline where it goes under the dashboard. It stops at the bottom of the catalytic converter hump, and the carpet on the side of the hump under the seat is also dry. Also, no water at all on the driver's side floor anywhere.

 

I had to replace my angled, behind the pedals, foot area of my firewall on the driver's side plus sections of driver's front floor - the angled foot area was fine on the passenger side except for an area the size of a quarter but I still had to do sections of the passenger floor - both sides of the cowl were rotted through around the cowl hats - my guess is maybe the heater box saved the angled foot area and diverted the water directly to the floor

 

Did you do all the work yourself?

 

Yup, I bought a mig welder, burned up a roll of wire and a cylinder of Argon mix and practiced practiced and practiced

 

It's not necessarily "hard", but it takes a LOT of work and some knowledge of how the car is built. I am still in the midst of putting mine back together. The hard part is finding what you are going to fix it with. I was lucky enough to find a rust-free cowl from Craig.

http://www.mmb.maverick.to/showthread.php?t=92251

 

you would be able to see the water...but you still can't tell if it's the cowl or the windshield gasket... the gasket gets hard and cracks. sometimes you can see separated places in the gasket.
you need a dry day, then pour water down the cowl. make sure none gets on the windshield. try not to let it splash.
also as Dave suggested, check to see if the cowl is stopped up. the way I check is at night I shine a light into the cowl, you can see everything.

 

If you took the heater box out and the driver's side plastic vent box, you could reach up into the cowl hats and feel around and maybe learn something about the cowl condition. If those vent/heater items are out of the car, then you may also see water actually running out of rust through holes in the underside of the cowl or around the hats. The mention of a flashlight is also a good idea. After finishing off my cowl hat replacements, a flashlight came in handy to find a few more spots that also needed attention

 

I bought a Mav. once cheap because the cowl was leaking. checked it at the Guys house with his water hose. got home pulled the heater box and found out that pine straw was up over the top hat and was wicking water in. pulled about 3 plastic bags of trash out, got my hose and washed the cowl out and ...no more leak...

 

also have done the ...flashlight trick...

 

Thanks for the help, folks. After a few days of heavy rain I've found the driver's side floor is wet too, so I'll hopefully be pulling the heater box out and if that isn't the case...guess I'm going to have to fix the cowls.

 

Once you get the heater box and drivers vent out, you'll be able to take a mirror up into the cowl and, with the help of a flashlight, should be able to get a pretty good idea where you stand with the cowl

 

With that real heavy rain plus plugged up cowl drains, you might get lucky and the rain water is only rising in the cowl high enough to spill over the hats - that is what is called positive thinking relating to cowls