Just some unreliable, off-the-cuff ramblings on the matter:
It's my understanding, generally, that where a bailment is created (generally, a situation where one is holding another's property) that mutually benefits both parties (e.g., pay parking lot, or perhaps a dealership doing warranty work), that the bailee (the one holding the property) owes the bailor (the owner of the property) a "duty of reasonable care" in safeguarding the chattel (the property). If so, then the issue is, at least under this analysis, did the dealership fulfill or breach its duty of reasonable care in the protection of your Viper? One might successfully argue that given the sticker price of these cars, storing it inside at night (which is the practice of my dealership) and out reach of vandals and thieves is necessary to fulfill the duty of reasonable care owed to you, and, conversely, storing it within reach of vandals is a breach of this duty of care, hence making the dealership liable to you for your damages. It would be interesting to know what the other dealerships do in your area with respect to storing expensive cars, not that that would be determinative on the issue of breach of duty, but it might give you some ammunition.
Additionally, given that the car was in their exclusive control when the damage occurred, it might be argued successfully that, under the principal of res ipsa loquitur (the thing speaks for itself), there is a rebuttable presumption that the dealership was indeed negligent so long as such things do not ordinarily happen in the absence of negligence. This principal has been used to put airlines on the defensive in a crash situation and may not be applicable to your situation. Again, just rambling here.
Also, there may be premises liability arguments and other arguments that could successfully be made.
Finally, and most importantly, I claim to know nothing of ILL law, so don't rely on the validity of any of the above. My blabberings are only to suggest that you might consider consulting an ILL attorney, which by the way, is my only actual advise to you. Just a letter or a call from him or her might just do the trick.
Good luck and sorry to hear of your misfortune.