There are several easy and inexpensive ways of removing permanent market from your windshield.
Some ways of removing permanent marker off your windshield are with the use of WD-40, rubbing alcohol, or acetone-based nail polish remover.
Either of those when sprayed onto or applied to the permanent marker on the windshield will help to remove the permanent marker.
After applying either of the above to the windshield and letting it sit for 5 or 10 minutes the permanent marker should easily wipe away.
Although sometimes you may have to scrub a bit with a sponge to help get the permanent marker removed all the way.
I personally have always just used some nail polish remover that I have on hand a lot anyway and it removes the permanent marker just fine.