It's difficult to plan for a vehicle fire.
In nearly any car, you have fuel of at least one kind, several oils, various plastics and insulation, synthetic and/or natural rubber, a lead/acid battery, at lease one kind of grease, accumulated contaminants, various types of foam used in everything from cushions to molded panels, upholstery of many varieties, and any number of items you may have placed in the vehicle.
Some vehicles are fitted with parts made of incredibly dangerous metals that will burn a hole right into the pavement if they light up.
Whether or not a fire can be extinguished without destroying the vehicle in the process depends entirely on the point of ignition and any surrounding conditions that may affect how it spreads.
Carry the biggest extinguisher you can. I use a commercial-grade extinguisher. If you've got a Co2 Tank, you have another option for pesky faults in difficult to access areas such as the dashboard or into the engine compartment before it's opened, and even a way to cool things down, but don't blame me if you fuck up with it and worsen the fire or hurt yourself.
You need to know how all of this works, such as when it's safe to open the hood or disconnect the battery. Sometimes, you know less than you think. I've seen plenty of firemen do some incredibly stupid things, and fires are their job.
If you aren't confident, just watch it burn.
The most important things you need to know are the precise condition of everything in your vehicle to avoid the issue as best as possible, and when to walk away.
Importantly, if you do manage to extinguish a fire, do not get back in the vehicle. It's just like a failed explosive device. Leave it alone for a while, and have it removed to a safe location where it can be disabled, observed, tested, and repaired.
Not all of this applies, of course, if you are in the middle of nowhere. Sometimes, you've just got to keep the faith.
Cheers,
Kennith