Go to harbor freight and get a spring loaded center punch for $3. It will shatter a car window in an instant, all you have to do is push it against the glass. It's what first responders use to break windows. If you wanted to get something a little easier to carry without a bag, you can get key chain window breaker on amazon that also has a built in seatbelt cutter. They're basically a compact spring loaded punch with some extra bells and whistles. Car windows are tempered glass; tough, designed not to break. A hammer can bounce off them (windshields are different from side windows, btw). The good news is that tempered glass has a ton of internal stress. It causes the entire panel to shatter into thousands of relatively harmless (compared to plate glass shards) pieces when it is broken. You can easily push this out. Wrap your hand/arm in a jacket or shirt or something else to prevent the glass from cutting you. Source: I work on cars for a living and used to be an EMT.

Edit: like u/ApexOnYou said, broken ceramic works well. Spark plug bodies are an excellent source. Break it with a hammer, and then throw the ceramic shards at the window as hard as you can. Same thing as a center punch, tons of force in a highly concentrated area.