A foam extinguisher, the 'spray' type that looks like a giant hairspray can. (Do make sure it's the vehicle type which is freeze resistant.)

I fully realize that this will only be effective against rather small fires. But I do not intend to fight bigger fires without protective gear. The car can easily be replaced, lives are not.

I choose foam because powder extinguishers will ruin the electronics. Insurance covers fires, not corrosion on electronics.