RAYE pretty much covered it. Funny how the spark plug thing gets around. Even across Internaional borders. Seems that the porcelin body of the plug doesn't make a lot of noise when it strikes the window making getting in the car less obvious.
SStrause, I just want to add to RAYE's comments about breaking the window at the corners. The importance of this is because the center of the window offers the most flex. Thus, if you strike the center, the blow is both absorbed and distributed thoughout the window and many times the window will not break. If you imagine throwing a rock into a lake, the rippling action would be the force of the impact being spread out from center.
The corners on the other hand are less flexable as RAYE stated. Generally, the bottom corners are used (probably because they are the easiest to get at if the vehicle is upright), but it you are upside down I'd imagine any corner would work.
As for the knife breaking the window, I don't know. But my guess is that yes it will work. In a panic situation, you'd have to be careful not to stab yourself. The point of a blade is not completely symetrical (as it is on a carbide tip), so if the tip of your knife isn't completely square to the surface it may posibly skip off to one side or another. Hopefully it just goes into the door frame if this happens. I've seen a guy stab himself in the leg using an overhead thrust. The guy missed the intended target, hit a hard surface, and the knife skipped off the surface and right into his leg. OOUCH!!
Another thing to consider is that when gaining access from the outside in, it's much easier. When you are the one inside the car flipped upside down with crap in the backseat, now in your lap, it's a lot harder to do even simple tasks. You have to consider, you may be injured, if the force of the impact was strong enough to entrap you in your car. My point is this, considering all these things, you may not be able to take a full swing at a window or even a partial swing to break it. These window breakers (carbide tip variety) don't require you to "hit" the glass. It doesn't hurt to stike the window with a little authority, but you can push the punch into the glass and it will still work. The tip has such a fine point that it easily cracks the window and the tempered nature of the glass takes care of the rest.
Hope this helps. I'm happy to help. Robert