Hopefully, you'll never be trapped in a car with the need to break the window from the inside. However, if you do find yourself in that situation, your salvation may be right behind your head. Here's how to use your car seat's headrest to break your car door window.
You don't have to speak Japanese to understand what's going on in the video above — it shows you what you need to know in a couple of seconds. Just take your headrest off the back of the seat, then push one of the pegs from the headrest attachment down into the space where your window retracts, just where the seal is. Jam it down in there, and then pull the headrest towards you. Doing this flexes the window glass laterally, and the result is a break, usually at the other end of the window. Since it's usually safety glass, it should shatter and crumble, falling away from the door (although not completely or in several pieces).
This works because automotive glass is built to take significant direct impact, perpendicular to the plane of the glass, but if you flex it or hit it along the edge of the glass, it's much more fragile. This method will work in a pinch or someone else's car, but for your ride, we'd be remiss if we didn't suggest keeping an emergency hammer and seatbelt cutter like this one in the glove compartment or under the passenger seat in case of emergency too.