I don't think there is a good fire extinguisher for cars. Too much fuel, too many flammables, too much heat.
(This is the best). What you want is Halon but, I would want at least 10 pounds of it. 20 years ago I helped save my (military) site when the power plant caught fire using Halon. We would have lost the site without it. Check out FIREBOTTLE race car systems: A 10 pound bottle is about $500.(!!!) 5 pounds is enough for to give you a bit of time to get out of the car - it won't save anything.
(This is the recommendation) Second choice (and reasonably priced) is traditional dry chemical extinguishers. Get the one's rated for cars. They run about $30ea for a 2.5lb extinguisher. Get two. Put one where you can get to it in the cab and the other in the trunk.
Stop. What ever you get make sure it is for auto's. Your car interior may get hot and some of these following extinguishers will explode from summer heat.
New to the market is a 1 pound extinguisher like the First Alert AF400 or $10 MAX Professional 7102. Looks like a bug sprayer. I think it would fit under the seat. These type are cute, and from reviews, they may have quality issues. Make sure it is rated for cars.
Since it is a few years since I got new fire extinguishers, I think I will get a half dozen of these 1-pound types and put them around the house.