2003 V6 Convertible leak - from firewall?

Hi all, question that I can't seem to find anywhere on the net for my model year. 2003 V6 Base Mustang convertible. Last year I noticed a huge puddle of water behind drivers seat. Quick research led me to a simple fix that seemed to work, use compressed air and blow out rear drains. This year problem still existed but on drivers floor, carpet was perma wet due to excessive rain. I pulled out drivers seat, lifted carpet, and sealed the entire floor seam. Next day it still leaks! I check out the insulation on firewall behind pedals and it's soaked like a sponge. Any tips as to how to find a cowl leak on a 2003? The car is otherwise in great shape, stored winters and pretty low mileage at 144,000 km. Thanks in advance for any help!

Thanks, good tips! I'm starting to think I've got it beat. I sealed the floor. The next day the floor was all wet and I assumed firewall, but later I saw significant water dripping from the under padding when I lifted the carpet to inspect for leaks. The underpadding behind the pedals was only wet the bottom inch or so leading me to beleive it was from the floor seams. I should know more tonight.

I believe the floor pan seals were the culprit. I sealed them, dried out the carpet and underpad. Then there was some rain and me soaking it with hose, no leaks. Put it all back together and washed the car, no signs of leaks!