Why do lights appear like straight lines on a windshield of a car? (becomes more prominent at sunset and night) 
i think thats the Diffraction of light caused by the scratches in the glass. or the wiper blade leave a pattern of dirt and grime in an arc. i hope someone can confirm my logic. 
my seconde question is why the light line become curved at bottom of windshield ?
 A: It is probably not diffraction unless you can see a separation of colours.
The most likely explanation is scratches on the windscreen (and the covering of dirt) caused by the wiper blades.
More prominent at sunset because the Sun is low in the sky and the windscreen is being hit by a shaft of light.  
 
You will note from the enlargement the absence of colour and the streaks visible adjacent to the shaft of light.  
A: The streak is caused by refraction (&/or reflection) from scratches on the glass or from streaks of oil, grime, wax, etc., on the surface. As Farcher said, it is not caused by diffraction. The reason the streaks are curved at the bottom of the windshield is because the windshield is curved, and/or because the wiper blade is not rigidly attached to the wiper arm, so the scratches are not precisely circular.
If you look more deeply into this phenomenon, you'll find that it's possible to "draw" 3D images using scratches like those that you've observed, by controlling the center(s) of curvature of the scratches at each point on a surface.
