I have had to use fire extinguishers on two seperate occasions. Both fires were in automobiles (one was an auto in a closed garage, saw the flames through the window). Used my house fire extinguisher on one. Used my car extinguisher on the other.

Keep it in an accessible, yet secure location. Once everyone is clear of the auto, use it as effectively as possible. Car fires are generally not super fast at spreading (from my limited experience) which buys you some time. The other obvious answer is to support other people who don't carry extinguishers in their car