Check the weather stripping underneith the cowl where your cabin air filter is. i had this problem too when i replaced my cabin air filter. didnt think it was important at the time.

 

agree with Tobian46, also make sure the cowl area is clear of trash debris like leaves etc it could also be your heater core doing that I believe??

 

Does your windshield get steamed up on the inside once the car has warmed up, and not clear, get worse, when you run the heater? If so, that's your heater core. I think there are 2 in these cars. One for each side. If not then it's the weather stripping for your carbon air filter. Also, if it is your heater core you will smell coolant in the car.

 

Check the topics "cabin air filter" and "ac condensation drain". That should lead you in the right direction.

 

Sounds like either the A/C evaporator drain elbow is clogged, or the air inlet grille panel (cabin air filter cover) may be leaking and passing through the heater blower. Both easy fixes. There is a modification you do to the air grille by replacing the original one with one from a Grand Prix. Can be found under "Permanent Cabin Air filter Cover Fix".