Problem with car dealer, any advise?

[TMagda]

Thursday my wife picked up her new Toyota Sienna at our local dealer. It was brought in from a dealership about 250 miles away for correct color, options, etc... This morning she noticed glass shards in the trunk area as well as some black glue in the back carpet. She called the dealer immediately to find out if the van had been damaged and after making a call to the dealer he sourced if from, confirmed that yes, it had. He said only the back window had been broken and it shouldn't be a big deal. I looked at the car when I got home tonight and found the shards are actually some glass and some plastic, that matches the tailight lenses, mixed together. Finding the plastic lead me to inpsect the tailight lenses more carefully, and sure enough, the gaps around both of the lenses are filled with shattered plastic. This makes me think the van was hit from behind hard enough to break the window and both lenses at a minimum. The salesman said he would have the general manager call us back immediately, but then they both left for the day. I got online and put a stop payment on the downpayment check. Do you think I was within my rights to do so? I still want a van, just not that one. I tend to keep cars for a long time and I would be concerned with long term perfomance of the paint, as well as the operation of the electric door. In addition, how much faith can you put into the repair if the guy who changed the tailight lenses couldnt be bothered to clean out the original broken plastic. This car was not cheap, and was only slightly discounted. I know my dealer could not have known it was damaged, but it's on him to make it right and he 's off to a bad start. BTW it was supposed to only have the 250 delivery miles on it but when it arrived it had more than 450, indicating a some use prior to delivery. Any opinions on how to proceed would be greatly appreciated.