First off, an airbrush helps.
A 'dustcoat' is a coat (usually airbrushed) of paint that is the colour of dust (or close enough) that is very thinned. I use Tamiya Buff thinned approximately 4 parts water to 1 part paint. Close enough to a wash. This is 'dusted', ie sprayed from a longer distance and very quickly. THe paint particles will settle on as a layer of dust.
Now, to get the windshield wiper outlines, take a piece of masking tape and tape it down flat on something you can cut it on. Take a protractor, stick a pencil or whatever into it, then measure the distance from the wiper where it connects to the window to the innermost wiping portion. Draw this arc onto the masking paper. Then measure the distance from the wiper to the outermost wiping portion. Now, using the SAME center as the previous arc, draw this second one as well. This will produce the arc of the windshield wiper. Cut the shape out, and stick it onto the window of the model. After this, apply your dustcoat over the whole model. WHen the dustcoat is dry, take the masks off, and you'll have the windshield wiper portion clean. If you want to add streaks, you can do this with a brush.