Sometimes drugs will be found in a car with several passengers. It's not uncommon for everyone in the car to be arrested. Under the legal definition of possession, more than one person can possess. The question is whether it was subject to your care, custody or control.
This is how people can get charged with possessing something that it's in a car when there may be several people in a car. Remember that the job of a police officer is to just see if there's enough evidence to charge someone with an offense, not necessarily enough evidence to establish guilt so it's not uncommon in drug cases.
Just take the case of marijuana possession, for example, if there are three people in a car and there's marijuana that's visible, that all three are going to pick up a marijuana possession charge. You may be able to successfully argue that you weren't in possession and that somebody else was, but you're probably still gonna get arrested for it. So, when you're looking at the question of possession, remember it doesn't mean ownership, it doesn't mean actual physical control, it doesn't mean that it has to be on you. You can still possess something, even if it's not on your person or in your clothing, or if you're not holding on to it.