The A/C also dehumidifies, removing the moisture and, one hopes, the fog.  I
don't know what's special about re-using inside air in your case.

Push the AC button and turn the temp all the way to hot. The reasoning
behind this is obvious.

The AC removes the humidity.  For the defrost to work you need the AC on.

You can turn the heat up high with the AC on and the problem should be
solved.

Natalie,

Most cars automatically run the A/C compressor when in the defrost mode,
even if you don't push the defrost button. If you want to see if yours does,
push the defrost button with the engine running, open the hood, take a look,
and see if the compressor is engaged. BTW, even cars that run the A/C
compressor in the defrost mode, don't run the compressor if the temperature
is below 40 degrees F or so.

Which vents are you talking about? Do you mean you switched the system to
the recirculate mode, or that you literally closed the vents inside the car?
If it is the latter (you closed the interior vents), then the defroster
worked better because you forced more air out of the defroster vents by
blocking off the other paths for the air.

Where did you set the temperature? Cold air is not as effective at removing
moisture. I can think of a few potential factors that might have made your
problems worse or confused the issue:

1) The HVAC Mode Door is not properly adjusted (this is the door in the
system that direct air to different areas - floor, vents, defroster). As a
result insufficient air is directed through the defroster vents.
2) The A/C  evaporator case drain hose is plugged, leaving water in the
evaporator case, resulting in the air from the defroster having excessively
high moisture.
3) You didn't wait for the engine water temperature to get hot, so you were
blowing cool moist air onto the windshield - this won't do a good job of
defogging the windshield, however, by closing the interior vents, you
directed a lot of air onto the windshield which helped.
4) Your problem was actually moisture condensing onto the outside of the
windshield (you said running the wipers helped). The only way the defroster
(defogger) can clear the outside of the windshield is to heat the glass up
enough to evaporate the moisture condensing on the outside. For this
application, it is irrelevant whether the A/C is engaged, or whether you are
recirculating interior air. The only thing that matter is that the air is
hot. I suspect maybe the reason the defogger started working is that while
you were trying things, the temperature of the air increased and that by
closing the interior vents you directed more air onto the windshield, which
warmed it up enough to evaporate the moisture on the inside and outside of
the windshield.

In general I think you get the best results from the defroster (defogger
really) with the temperature control on full hot, and in the exterior air
mode.

What did you mean when you said - "The button for defrost clearly says to
open the vents." I've never seen anything like this. I suppose they use the
interior vents to defog the side windows.

What's wrong with your rear defroster?  If it is a broken "wire" in the
grid, you should look at the products available from
http://www.frostfighter.com/ .

Regards,

Ed White

Because you don't understand what "air conditioning" really is.

Boy! That must be some big button. The buttons in my car have symbols on
them, not words.

You were pulling in cold, moist air that condensed on the windshield. In
my car, the A/C goes on at about 50 F when it is on the defog setting
(to dry the air going to the windshield). Below about 50 F, there
usually is not enough moisture to fog up the window.

So my guess is that when you closed the vent, the air around the
windshield warmed up enough to let the moisture evaporate from the
windshield.

Jeff

Reusing inside air is about the worst thing you can do to lessen fogging of
your windows. Turning on the AC helps, as it dehumidifies the air.