Recently I bought a Luminous Emergency Hammer and Seatbelt Cutter for my car. But there are also Pocket Size Car Rescue Tools, designed to go on a keyring. The most common model is the ResQMe Keychain LifeHammer. These have a spring loaded steel pin which is fired when the device is pressed against a car window, to shatter the glass. There is also a blade recessed in a slot for cutting a seat belt.
While these tools are designed to go on a keyring, perhaps it could be attached to a Heavy-Duty Retractable Key Chains Reel
clipped to the car. That way it would be on hand when needed. I noticed that the escape hammers in buses are similarly attached to steel cables to stop them being lost (or stolen).