Lets see... I notice that "Love Potion #9" is available....
(sorry about that.. couldn't let your spelling error go to waste!!)
Seriously, if they are denying liability for the cracked windshield, your options are limited, in my opinion, to:
1) Writing them a letter, certified RRR. Spell out your case that they are liable and ask that they accept responsibility for the repair. If they don't then you will need to:
2) File a suit in Small Claims court for your 'damages' (the reasonable cost for repairing, if possible, or replacing the windshield).
This will become a 'you say, I say' case... and the requirement is on you to substantiate your claim that they are responsible. Just a "it wasn't there before" statement won't work. What evidence do you have to support your claim that they are responsible for the windshield damage???
(Since your post said they were replacing the wipers, I would assume that any prior damage would have been noted on the work order???).