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 Suppression

Amerex Custom Fire Suppression

Deist Safety On Board Fire Suppression

Fire Fox

Safecraft 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.