If I am not mistaken, once you sign the paperwork the dealer does not have to take the car back, depending on the situation most will. Do not be misinformed that the dealer ship has to take the car back. This information comes from and an attorney when I wanted to do the same thing because I thought the truck I had purchased had been in a wreck.
The Dealer can tell you "tough luck buddy" if they are inclined to. You are basically at their mercy so use your manners and don't piss them off, Good Luck