If you know anything about modeling.. well actually I take that back, if you can read you should be able to do this.
Select the car, and on the right panel choose "Element", and click on the windows, and then down that list on the right side you'll find "smooth" all you have to do is just make it set so it smooths it, the value should be fine for what you're doing. I don't remember what the button says, but it should be pretty obvious, I'm to lazy to run it and figure out what it says ATM.