When I used to be in the State Emergency Service and attend vehicle rescue, we had a life hammer we could grab easily. I now keep one by my handbrake lever.
http://www.lifehammer.com/

I also have a ResQMe on my keyring because I quite often find myself in other vehicles.
http://www.resqme.com/

The ResQMe is a spring loaded centre punch, but the tip is protected by plastic and it has a seatbelt cutter, and the Life Hammer is used like any hammer, but if the seat belt jams, the extra cutter on both is a good idea.