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How to break the glass of a submerged vehicle using the headrest(r/LifeProTips)
Truly a LIFE pro-tip.
However, if you are able, please keep a window-breaking tool conveniently accessible in your car. Either a hammer or spring-loaded multi-tool. Just make sure that whatever you get can also cut seatbelts (like the two I listed here).
Mythbusters did a couple episodes on escaping from an underwater car. After their experiments, even they said they would be keeping them at arm’s reach at all times while in a car.