I have a 2003 impreza rs and the windows fog extremely bad.

 

Use heater temperature control in conjunction with air-conditioning or defroster setting to de-humidify the vehicle's interior. Search of forums yields this existing thread.

 

not trying to be an *** but have you tried cleaning it..I have noticed my windshield fogs more frequently when it's dirty.

 

my 2005 is really bad!
as soon as i turn the defroster setting off my windows will fog up almost instantly!
i have been experimenting though. last time when i took my car for a longer drive
on the highway i had the heat blasting all the way and I had the windows cracked open just a bit to let the heat out. it's a little better now but still....

 

Are you getting an Anitfreeze smell in the car as well? If so go have your heater core replaced?

 

no anitfreeze smell, i put an end to fogging up though!
I did again a good highway run (about an hour) with my heat
all the way up and windows cracked open a couple of inches -
now i got no more fog up issues!
it worked!

 

check your circulation settings. If the air is on recirculation (arrow going into car and looping around inside car) then it will cause the windows to fog up quickly. if you use the non-recirculation setting it should work

 

what he said...up above.
pluss you could even use ac + cold air..doesn't have to be hot??

 

x2...also check for moisture already in the car. my volkswagen did this when i had a puddle in the back seat...i know it's a different car, but the concept is the same

 

My a/c doesn't work (another story) and thus niether does my defrost, so I usually drive with my windows open during rainy weather. Someone suggested I use an anti-fogging product on the inside of the windshield. I tryed it out and no more problem! (at least as long as the product is working).

 

check for vacuum leaks?

 

i just so happend to be looking in my owners manual for something unrelated but stumbled upon somthing that might help this problem. it says if you are having fogging issues then replace your air filter for the ventilation system(its behind the glove box)

 

agree because when getting in and out of the car while its raining the interior carpet will get wet and the heat along with the wetness on the floor can cause the windows to fog up.
running the a/c compressor will remove the moisture from the air and you can still be warm.