hi, i own a 89 civic lx with ac. Every winter the windows and the windshield always start to build moisture and fog up when i run the heater. Its not a coolant leak because the moister is not greasy or smell weird. Does anybody know the reason why this problem always occurs with the moisture in winter ?

 

Condensation happens when different temperature meets, unless you turn on the AC (partial, like windshield defrost) to dry up the moisture, you will get some condensation within the vehicle until the temperature neutralizes.

 

turn on the a/c when you're running your heat and it will take dry up the moisture just like charles said...

newer civics have the a/c turn on automatically when you select the defrost option... i'm not too sure how it works on a 4th gen...

 

why does honda cars need a.c for windsheild to defrost

 

The A/C removes the humidity from the air and dries up the interior.