Forget portable fire extinguishers to put out fires in your OWN car (you'll still need them to help someone else). Take a look at these (quick google search "vehicle fire suppression"):
Crown Victoria Police Interceptor Fire Suppression Levitt-Safety Custom Fire SuppressionAmerex Custom Fire SuppressionDeist Safety On Board Fire SuppressionFire FoxSafecraft Vehicle Fire Suppression Systems Note: Several webpages indicate HALON is dangerous, and should not be used in confined areas where someone may breath it in.