I recently had a Vintage Air Surefit system installed in my '69 Malibu. The A/C works great, as well as the heat, but the defrost setting does something strange: When I first turn it on, the windshield instantly fogs over, then dissipates within seconds from the hot air coming through vents. If I turn the defrost off and try it again right away, no fogging. But if I turn it off and wait five minutes then turn it on, the windshied fogs over again and then quickly clears.

I checked the carpets but they aren't wet. And I don't smell antifreeze when this happens. Anyone have any idea what is causing this?

does it fog on the inside or the outside? Cool air coming out of the vents will chill the surrounding air to below it's dewpoint, and the water will condense on the nearest available surface. The air blowing on the windshield will chill the windshield which will chill the layer of air nearest the window on the outside, dropping it below its dewpoint.

I would check the evaporator case drain to make sure it is not plugged.

The VA system is setup to turn on the A/C when you hit defrost. This makes the A/C evaporator core cool which condenses moisture out of the air passing through (cooler air can hold less water). This dried air is then heated by the heater core and passed over the windshield. Being warm/dry the air can again hold more moisture and it drys the fog off of the windshield.

This process takes a few moments to get started though. In the meantime whatever air is in the system will be passed out the vents. If this air has a lot of moisture in it, i.e. is saturated, then the cool windshield can condense moisture out as it cools the air. The air inside the system could have a lot of moisture in the air if water is accumulating in the bottom of the evaporator case.