It sounds like your extinguisher was to small to make any difference here. So If there is no one inside vehicle, just back off.
Car fires are very dangerous, not from just the heat and flames but the fumes and smoke given off are very toxic. for example car interiors can give off cyanide when burning, mechanical cylinders that control seat belts and some rear windows and doors that raise and lower can explode. shock absorbers can explode. Some automotive parts are manufactured from magnesium which can react with water violently.
The best thing to do is let the firefighters take care of it, they are trained to do the job.