Hi, SRS.

The optimum device is the spring-loaded punch. Place it in a corner of the glass to be broken and it will break. Keep in mind that automobile glass that has shattered (broken into little bits) will still be there as a window. The glass can cut a person up really badly, so ensure you use a well-gloved hand to get the glass outta the way. And be careful not to get shards in your eyes. Good way to cut the eye up.

A claw hammer will work or actually any fairly pointy object. We did, on occasion--use car aerials to snap across the glass to clear the windshield in order to extricate people. Any fire department will let you know that stuff, volunteer or full-time. The usual call is `Breakintg Glass` and boomp.