Discrete categorization of continuous phenomena is always going to be inherently arbitrary. It's important to keep in mind what your goals are. This sort of categorization shouldn't be done with the goal of finding some sort of fundamental platonic truth; that's not going to work. Rather, discrete categorization is performed simply because humans find these categories useful when communicating.
The fuzzy line between truck and car isn't a problem when you approach categorization with this mindset. If you see a bank robber fleeing in an El Camino, you can tell the cops they fled in a "truckish car" or a "carish truck"; you don't have to neatly categorize an El Camino as one or the other to get your point across, but the arbitrary categories still help you communicate the idea.
If you're tagging pictures and come across an El Camino, and you don't have a "coupe utility vehicle" tag, you can simply tag the El Camino as both a truck and a car.
The fuzzy line between truck and car isn't a problem when you approach categorization with this mindset. If you see a bank robber fleeing in an El Camino, you can tell the cops they fled in a "truckish car" or a "carish truck"; you don't have to neatly categorize an El Camino as one or the other to get your point across, but the arbitrary categories still help you communicate the idea.
If you're tagging pictures and come across an El Camino, and you don't have a "coupe utility vehicle" tag, you can simply tag the El Camino as both a truck and a car.