I want to be clear about something right up front: I am not condoning grand theft auto. Hot-wiring is a very useful skill, but not so you can steal cars.
However, after the SHTF there could be situations where you desperately need a vehicle to escape danger, go on food and water runs, get an injured person to someone who can help, etc. If the original owner of the vehicle has abandoned it or has died, and if you’re living in a post-apocalyptic world in which no one will ever sort out the proper owner of the vehicle, then I think it’s perfectly acceptable to take it.
The following article describes two ways to start a car without a key: the hot wire method and the screwdriver method. In case something goes wrong, I wouldn’t practice this on the car you use everyday. However, if you have an older car around that you don’t rely on, this skill is worth practicing.