Dog in the car wont work - cats are fast learners and would simply tramp forward and backward in front on the window, to seriously piss the dog off.
Of course intelligent dogs would either open the door (might be uncomfortable for the cat) or (not so intelligent) crash through the window (might be uncomfortable for the dog).
Perhaps you could attach a battery to the car? Small shocks might be just annoying enough
(and no, I'm not seriously proposing this).