You need to examine up under the dash board looking for dampness and rust.
Suggest you remove the cowl and examine the ventilation air intake area, seams for rust and separation.
To remove the cowl you first remove the windshield wiper arms (slide keeper outward and lift arm), remove the screw in the center where the ventilation grill is mounted and then the hood to firewall seal, two screws behind the seal and the other four screws. The cowl should lift out. Do a thorough inspection and clean any trapped debris laying inside.
Inspect the door seals to ensure they are still pliable and no separations are present.