Complaint Review: Enterprise Rent A Car

We were issued a rental car by our car insurance company, Allstate.  Another person had hit it and it was being repaired. During this time we noticed that the rental car had an odd smell.  Couldn't figure out what it was.  And found it a bit odd how they (the employees at the rental place) pushed the extra insurance on us so hard.  Well, it rained the day after we got this car, sometime between the day after it rained and the next, we noticed a large amount of water on the drivers side, on the bottom of the car.  It went from the front to the back, pooling in the back.  We called Enterprise and told them that this had happened.  They said they were going to make a ticket and we should go to the nearest Enterprise and get a new rental when they are open (they were closed because this was on a weekend and after hours).  Monday comes around and we turn in the car to get a new one.  The employee informs us that we will be charged for the damage to the rental.  We didn't cause this damage and it seem to be a preexisting problem.  Our insurance company tried to fight it but said that since its not bigger than our deductible (they are charging just under 500$) that there isn't anything they can do for us.  Now we got a packet in the mail that says we need to pay them with in ten days.  Nothing, not even a phone call before this.  They have no proof that we caused the damage and we have no proof that they caused the damage, yet its ok for them to accuse us and go on with charging us?  I'm very sure that this car was previously damaged and they knew it.  Its not something you would know just by looking at the car.  How do I fight this?  They actually claimed that we "left the window open" when it rained.  When we argued, they said that we must have driven though a "puddle" when it rained.  A puddle, like you can drive and not hit one when it rains?!  PLEASE HELP!