What bookwormsarah said. :-) If you've got all the pieces, I'd clean them and use superglue. And some sealant if that's necessary to make sure there's no fissure left for water to enter through. Also, if you car is more than a year old, you can probable find a spare at a ... erm, what on earth do you call it? One of those places that recycles cars. Search online for the "glass" (which probably is plastic). Some handsome handy guys around to ask for help, maybe?