toothpaste
only use the plain paste kind, not anything fancy, just the 99 cent variety
spread over the scratch, let it sit for a few seconds
wipe it off
DO NOT RUB
this is a wipe on wipe off type deal
do it a few times and it should clean up a bit
rubbing will score the window so be careful
try at your own risk
i did this to my rear window on a few very small scratches and it worked great, but it is risky and if you rub too hard it will scratch the shit out of the window
so start slow and be careful if you try it