You are parked, and someone leans toward the window to greet your dog. The dog growls. In a car, they cannot simply walk away, so an approach can feel much closer than it would on a path. Ask the person to step back. Keep the window closed enough that nobody can reach in and the dog cannot reach out. Do not scold the growl or invite the person closer to prove they are friendly.
Secure your dog appropriately for travel and follow local vehicle rules. Never leave a dog unattended in a car, heat and other hazards can become dangerous quickly. If they have snapped or bitten, keep others away and seek qualified, reward-based behavior help. A new reaction or discomfort getting into the car should prompt a veterinary check.
Change the picture
Park away from busy footpaths and choose quieter stops. If the dog is comfortable with a safe visual barrier that does not interfere with driving, reduce their view of close passersby. Practice outside the car first: at a distance where your dog can see people and remain relaxed, reward looking and turning back toward you. Later, in a parked car with an adult able to supervise, reward calm observation from a much greater distance than the original trigger.
If your dog stiffens, growls, or stops eating, stop practice and give them more space. Do not ask a stranger to approach a confined dog for training. The first goal is a journey and a stop where nobody has to put a hand near the window. After securing the dog and planning safe stops, the reactivity around people guide offers more on reactions to people nearby. Do not invite anyone to approach the car for practice.
Sources: Dogs Trust: Reactive dogs - why it happens and how to help. Dogs Trust: How to understand your dog's body language.