Make sure the recirculation mode is off (I dont think you can turn it on when in defrost or economy mode), this can really fog up you windows when it is left on long enough. Also if your a/c system is non-functioning and you still cant get the windows to defog, just crack a window about 1/2" this usually works. If you are really having problems there is antifog stuff on the market, I have never tried it so I dont know how well they work. Another possibility is the a/c compressor isnt kicking in because it is too cold outside, the a/c compressor on my Integra does not kick in when the ambient temp is around or below about 35 degrees.

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Adam,
2003 Volkswagen Golf GL 24,000 miles. 1.9L I-4 TDI 5spd.

1986 300E. 115,000+ miles. 3.0L I-6 1st daily ride, around town only. (Gone)