how deep the crack goes depends on how much you have to sand off. a lot of times it's just a chip in the paint or surface, but cracking to where it's just minor can be filled with a good 2-part epoxy.
if it's a vertical or slanted crack, you can use a round file to remove the damage. sand the surrounding area about 2-3 inches from the damage so you get a good surface to bond, then filled with epoxy, or resin and fiber glass sheet.
pictures would help too. good luck, and keep us posted
don't forget to mask your rear window to keep whatever you're using from running down. avoid removing the window if at all possible. some have cracked or shattered theirs when reinstalling it.
-coarse grit and fine grit sandpaper, and/or file (remove/clean damage)
-2-part epoxy (offered in different color settings)
-mixing container or cup (anything but wax type)
-acetone (not nail polish remover, even though its main ingredient is acetone, but seems heavily diluted) or equivalent
-newspaper or equivalent (to mask off with)