Condensation has to do with two factors, temperature and humidity. Specifically, in order for condensation to occur, you need a body of warm, humid air (humidity is necessary because otherwise there wouldn't be any water to condense) and a cold surface (the window), specifically colder than the dewpoint of the humid air, on which the condensation is to take place.
With a car window, this can happen on either side. In your case, you're getting condensation on the inside of the window because the interior air is warm and humid, but the outside air keeps the window cooler than the dewpoint of the inside air. So as long as you blow warm air on the window, you're just feeding it with more water to condense. In order to make the condensation evaporate by heating it, you'd have to be blowing enough air at a high enough temperature on the window to override the effect of the outside air and significantly heat up the window itself; your car's heating system probably isn't capable of doing that. (At least not while you're driving.) In contrast, as you pointed out in your comment, blowing cool air (cooler than the outside air) on the window means that the dewpoint of the inside air is necessarily less than the temperature of the window, so you don't get any condensation occurring. And as long as the air is less than 100% humid, any existing condensation will evaporate. This is especially likely if you're running the air conditioner, since it doubles as a dehumidifier.
The reverse situation occurs if it's hot and humid outside; you'll probably be running the air conditioner, so the cold air inside the car will cool down the window. Since the window is cooler than the outside air, you'll get condensation occurring on the outside. The solution in this case is to run the defroster to heat up the window and the condensed water molecules, giving them enough energy to evaporate.
It is called Venturi Effect.
The increase in speed of the air surrounding your vehicle comes with a decrease in pressure. That explains too why a chimney works better in windy days.
The Venturi effect is explained by applying the Bernoulli Equation (say, the conservation of energy of a small piece of fluid that moves within the flow) between two points along a streamline (in this case, we would follow a piece of air in a tunnel wind)
$\frac{1}{2} \rho v^2 + \rho g h + p = \text{constant}$
The increase in the first summand when the flow gains speed to adapt itself to the shape of the car, is compensated by a decrease in the pressure $p$. Look what happens in this picture (wikipedia) when the flow changes speed to adapt to the shape of the tube:
(Image from wikipedia)
$ $
Note the similarity with the high school equation for the conservation of mechanical energy of a particle:
$\frac{1}{2} m v^2 + m g h = \text{constant}$
(Just change the mass of the particle for the mass of a fluid volume unit, i.e. density, and add an additional summand to accout for the pressure, and you have Bernouilli's equation)
Bernouilli's equation is meant for an incompressible flow (water) which here means that the numerical results would be approximate, but qualitatively the same effect happens.
A related, interesting fact, is that submarine propellers must be carefully designed, in order to avoid points in which water suffers much too rapid a speed increase. When that happens, pressure becomes so low in that points that vacuum bubbles appear. The power released by the implosion of that bubbles against the surface of the propeller, not only is noisy, but also may damage the propeller itself. The phenomenon is called cavitation.
(Image from wikipedia)
Best Answer
While @Nick gave a good answer (“air flows up and around the car”), that answer by itself would mean no bugs ever hit the windshield - and we know that is false. So what’s the difference between a bug and a butterfly?
If we look at the problem in the frame of reference of a stationary car, there is an airstream moving towards it, and in that airstream there is a small solid object (bug, butterfly).
From the frame of reference of the object, it is in a body of air that suddenly moves up. The question then becomes - will the object move with the air stream? This depends on the size and strength of the wings and the mass of the object.
If you are a bug with small wings that you have to beat very fast to stay in the air, then most of your “lift” is generated by the motion of your wings. If the air moves a bit faster, it won’t change the lift you experience by much (because your wings were moving so fast to begin with, the extra speed of air over the wings is small). So you will go splat.
If you are a butterfly, you get enough lift without moving your wings much (because the wings are big). So if the air starts moving faster, it will tend to carry you with it.
Lucky quirk of evolution - small body with big wings will avoid fast-moving objects (though I am pretty sure that was not the main reason why butterflies evolved to have large wings...)