[1986nate]

All cars turn on the compressor on the defrost setting...By using the freon almost all humidity is taken out of the air that comes in which in turn gets rid of the "fog" quicker. Also other cars that may have a setting for air by the feet and defrost at the same time, most will also turn on the compressor for that. However in my 95 cutlass when on the feet/defrost setting it would not turn the compressor on and so I never turned that car on full defrost. Just something to check out.

Edit: I should say every car that I know of. There may be a slight few that may not work this way.