Perhaps the most important thing first. We'll assume that fire breaks out in your own vehicle. Quickly find a more or less safe place to be and ensure that the safety of any passengers is also guaranteed. Of course, you should get away from the vehicle but not before you have released the bonnet-clamp and switched off the ignition.
At this point there are several possibilities. Should you have an extinguisher at hand, you may be able to contain the further spreading of the fire. With less than two kilogramms of extinguishing agent however, the chances of success are very slim. Don't just blast away in panic, rather attack the fire with intermittent bursts.
If possible, it makes sense to somewhat expose the fire. Indeed, you should remember that a great amount of heat may develop and any parts may be very hot, even it's only a smoldering fire. It is important to protect your hands, e.g., when lifting the bonnet. With any luck, other drivers may come to your aid with their own extinguishers. The danger of explosion is considered by experts, to be relatively low.
Of course, the whole action should not take place without fire alarm being raised. Particularly the smaller extinguishers only have a very limited volume. Even if the car in question has to be abandoned, the combustibility of the surroundings must be taken into account. Should a fire break out on or in the trailer, it also makes good sense to attempt to save the towing vehicle. 06/12