If the portion of the windscreen or side windows through which you are shooting are parallel to your lens/camera focal plane your image should be ok, outside of dust/dirt on the window as referenced above...... if you are near the curved edges of the windscreen and/or shooting at an angle to the plane of the window, will tend to cause the light to refract and create blurring/out of focus effects .
If it gets too hard, and you have enough time and a spare hand, you can switch to DMF and focus with manual override.
Possibly using the magnifier as well.