Earlier quoted context omitted.
My wife's cousin almost died from heat-stroke in her drive-way when her car trapped her inside the vehicle in full Australian summer sun because the battery failed while she was inside checking something. Doors and windows wouldn't open. Nobody heard her calls because the new car had good sound-proofing. She eventually managed to crawl through the panel behind the back seat to get into the boot (luckily she's quite p…
It's too late now for her of course, but you can take the headrest off the seat and use the metal prongs to smash the windows to get out.
Somebody on YouTube tested those emergency hammers and concluded they're basically useless. If you really want to break a tempered glass car window (I.e., any of them but the windshield), their recommendation was an automatic center punch.