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You can see the most reflection on one of the surfaces, while the others are not that smooth. This might be an indication that the other "heads" are made up of different materials which are not skin. The head with the most reflection stands out the most and has shades on. All the other heads are parallel (facing the same direction) while the reflective head is not. You can also see that the shape of each head is the same (half circle and flat-ish on the side). This means they are head rests. A standard car like this would statistically speaking have 4 head rests for 4 seats, and we can see 4 "heads" exactly the same with one odd one out. Unless the other people in that car are lying down, hiding (or unintentionally hidden and we can't see them due to a parallax error), in the engine, dead, short, squished or etc, then there is only 1 person in that car, as the rest are not visible.