I found some blades of grass that had started to grow in my car, so i took out the driver seat and pulled up the carpet to find the 2 compressed fiber mats just drenched. Even the part that extends up behind the pedals was soaked. My father suggested maybe the windshield has a leak near the base of it or something. Any ideas?

 

That seems a pretty good guess.

 

if the windshield has been replaced, its possible for it to be leaking, in the bottom corner of the drivers window it'll say either toyo kyogo or Mazda if its original.

other places to look, are the sunroof drains, if you have a sunroof.

also the seal around the door, and how tight the windows close also should be looked at, and also the drains on the bottom of the door, and maybe the rear hatch too

 

My old car had water coming through the firewall, where the main wiring harness comes through. The rubber grommet was so old it had disintegrated and left a gaping hole near my pedals.

Something to check.

 

i checked the passenger side earlier and found a round chunk that was wet in the front foot well, back passenger was dry and i had about an inch of water in my spare tire well... the window isn't leaking, and the main wire harness grommet is still good and tight.

 

Water in spare tire is usually rusted out rim under the hatch rubber seal and/or rust holes around the clogged drain holes.
For the front floor...
Remove the grill piece on the outside of the car in front of the windshield (cowl). Look down inside towards each side of the car to find the vent intakes. Lots of cars have rust holes around these intakes. Water is supposed to drain over the side of the cowl area behind the fender. With holes around the vent intake it can drain into the interior. Hopefully yours doesn't look like this one did.

 

If you dont drive the car its going to be some weather stripping some where have some one get in the car and you spray it with a water hose and see if you find the leak that way

 

I also vote for the window seal and water hose test. You living in the rainy Northwest you need to get this sorted asap. Be concerned with mold build up also.

 

Update: I found three leaks in my car, the door seal is leaking in the corner where it turns (by where the windshield meets the roof.) the water is running in the trough above the door that goes through a hole by that spot that leaks down behind the weatherstripping. Its like that on both doors. Im going to take some silicone and fix the hole in that trough so it cant leak behind the stripping. Then i have water that is actually coming in under the trunk seal...not sure how its doing that....maybe a small hole in the limp that the trunk seal sits on.....hoping these are the only spots where its leaking...im tired of fogged windows and ice on the inside of my windshield.

 

My guess is that the drain holes for the hatch were clogged for some time and the lip is rusted out. If the holes are small its not so hard to fix. If they are big its gonna take some welding and patience.

 

also, check the front wheel wells. there is a seam that often rusts and allows water in when driven in the rain. the water seeps in unnoticed and weeps into the carpet and the sound deadening material. eventually over time, you will see plant growth and mildew.

 

If you have a rear windshield wiper, I have found water comes in through the seal / grommet that the motor shaft goes through that is connected to the wiper. The cover fills up with water and splashes out into the carpet and then slowly fills up the spare tire well over time.

 

So the wiper leak fix would be new gromit I suppose or is there something else.
I also have old school louvers.