Leak in floor board,drivers side
 

The floor board on drivers side always stay wet under mat and where heal would rest. It’s about a Four inch diameter. It started smelling bad after Toyota dealer changed the cabin air filter. Anyone know what could cause this

Ok; more info please? Firsr I would check the cabin air filters, maybe there is a chemical treatment that you are smelling. or they were clogged and now you have airflow over rotting carpet up to your nose.

Its very possible you have a hole in the front windshield gutter and it is dripping on your feet. My thoughts are to pull the front seat and with a powerful light from your backside search for the hole under your dash. Patch it with glue and a plastic dinner plate or some such to stop the drip.

Have you checked the floor boards? unscrew the trim and lift the carpet to see if you are Fred Flintstone in disguise. You cut, weld, glue, a patch panel in. I suggest hush mat be installed on top to form a water barrier to protect the metal.

I didn't know a Taco 96-04 or a 4R 96-02 had a cabin filter, so my first question is what vehicle are we talking about, and then I would suggest putting that Q in the correct section, and a stink doesn't occur over night, and the cabin filter is on the Passenger side in all American Domestic Market vehicles.

Pls see signature.
You are in the best position to see, smell, feel what and where that moisture is coming from.
Feel, smell and see the moisture. Is it water? Oily? Others?
Insect the area and above it to see if anything drips.
Carpet gets wet even when there's no rain? Even when air conditioning is not running?
Which heel? Left or right? Do you have auto or manual transmission?