A/C conditions the air (dries it) so the defrost will work better... it does work without a/c but only on the heat setting, and it's fuking miserable to ride around for example when it's raining and wet outside and have to have the heat all the way up to use the defrost... Try this - next time your windows get fogged, turn on the defrost and run it for a few mins without the heat turned up... next, hit the A/C button and watch the fog start to disappear... actually most a/c systems now days are linked to the defroster to kick on whenever you select defrost... also, if you keep it on recirc instead of fresh air, you're just recirculating the moist air in the car instead of fresher, drier air from outside and that's why it doesn't work as good and why on some cars, you can't even select defrost and the recirc/max mode @ the same time... hope this helps a little