Has anyone had a problem with water getting into the passenger side floor after a hard rain? I have a '05 GT.

 

Tons of information here about that topic; it was a known issue on some '05s. Take a few minutes when you have some time and search the forums, you should be able to find info.

 

If you take the plastic cover off under the windshield wipers, and reach down past the cabin air filter, there's a rubber drain down there that can get blocked by debris over the years. As it gets loaded up with water after a heavy rain, the water will then run thru the air filter and down onto the passenger side floor on inclines. It's an easy fix. If you search the forums or google you'll find how to get the crap out of the drain.

 

easier way to clean(both sides have them) : https://themustangsource.com/f739/co...-clean-504787/
be sure to read post 4 about the padding under the passenger side carpet- I had to cut mine out, it was still sopping wet 4 days later, even with the carpet folded up/big orion industrial sized dehumidifier-fan blowing in... theres a rubber layer between padding and carpet, if you dont dry that pad,the carpet will feel dry as the floor rots...dunno why drivers side was rubber padded, it dried overnight with carpet folded up- that passenger side will never dry without removing.

 

Also the cabin filter should be ready for changing about now anyway.