Always have an emergency tool in your car to break your windows in case this shit happens to you. Even if you're in a car not designed by a god damn moron.
Also to break glass, don't hit the middle of the glass, strike near a bottom corner where it is less likely to flex against the impact. You may be able to take a seatbelt's end and use the corner of the metal end of the seatbelt to break it if you have nothing else. Corners for breaking
Well, any seatbelt still has a metal point or two, which can be used to break the window.
But the person inside still needs to be conscious and not-panicking enough to realize that fact.