mildew in car

Check the air intake vents right up in front of your windshield and the heater/AC air delivery hoses.

When it rains water runs down the windshield and into the vents. There is usually a drain to allow water to escape but if that gets plugged up the water can back up and get into the interior air vent tubes and sometimes it leaks out onto the floor (usually on the passenger side).

You can also get water on the floor if the A/C coils are plugged up or the heater core itself is leaking (although that last one should smell like anti-freeze).

Make sure the vent drain holes are clear. If that doesn't do it you might have to take apart the heater box under the dash on the passenger side and clean it out.

mildew in car
stop driving on pavement!

labrat, the first car i ever owned has a leak on the front passengers side. it never really had a chance to dry out-- one day i went to the car ad discovered tat mushrooms were growing there, i thought it was pretty great, although that's just me.

GMs have consistent problems with leaks like this. As far back as 1980 it started becoming an epidemic. The only GMs I own that don't leak are the three I have that were built before 1980. The most common cause is a carpet alignment issue. Pull up the door jamb trim strips. Sometimes the carpet/padding extends past the seam and allows water to wick up from the door. Sounds unlikely, but its common. Saturns in particular sometimes leak where the A/C housing attaches to the firewall. Check for leaks there. The good news is that water tends to wick pretty evenly through the padding, meaning that you can often tell the source of the leak by where the water has wicked after a rain. If you pull the sill trim and reach your hand under to find that its wet only one foot in every direction from where you stick your hand in, chances are its leaking right there. Also check if the saturation is different after a rain when driving and when its just been sitting still.