I agree with @Barry- the types of fire extinguishers people keep in their cars are mostly absolutely useless, hence the quote from Gaurav's post.
Moreover, once a car catches fire, it hardly matters what caused the fire, and debating the percentages of possible causes becomes a useless exercise. I have like @Barry seen cars catch fire, and in my experience, the only thing you can do when your car catches fire is to get out of it, and away from it as fast as you can- taking only your children with you if they are in the car.
Doing something stupid like opening the bonnet to see what caused the fire only feeds the fire with oxygen, so don't do it. If the fire starts in the passenger compartment, and the windows are closed, get out, and shut the doors to starve the fire of oxygen. If the windows are open, get out, and get away as fast as you can- again taking only your children with you.
In short, once a car fire takes hold there is nothing you can do to stop it, but Gaurav's post contains some good advice on how to prevent your car catching fire- except the bit about car fire extinguishers being effective against electrical fires. The only time this might be true is when you can actually see and reach the fire, which is not possible if the fire starts under the dashboard, for instance.