Hit it in the very corner. You can hit car windows (even door windows) with a hammer, tire iron, gun, etc dead center and it won't even crack. It is pretty funny watching someone in training do this with a [unloaded] pistol for the first time.
But, if you hit it in a corner it will give. Another fun thing to try if you ever decide to start a new profession as a car thief - walk up to a car and snap off the radio antenna and smack the back lower corner of a door window......instant access.
I tried breaking a side window with a tire iron once, big mistake, all it did was hurt my hand.
You can easily bust out most any vehicle side window with a chunk of the ceramic from a sparkplug.