The shadow luminance is a function of the ambient light level, if no fill light is used. Play with those variables until you are satisfied with the objects' appearance. Highlights become brighter if you increase lamp brightness or move the lights close, and shadows become darker the less the surrounds reflect or emit. Cars are different, especially black ones. Glossy paint has a suspension medium that forms a smooth surface, producing specular reflection.
Glossy paints are more saturated than matte, except when reflecting light toward the viewer.
Similarly, the reflectance at the surface of a solid covered by water, wax or varnish is considerably less than if the liquid were absent. Contrary to highlights on matte objects, highlights on glossy surfaces are images of lights reflecting in them. Whereas lighting designers dealing with matte objects are shadow designers, the lighting of glossy objects is highlight design. A similar problem occurs when lighting large gems, or porcelain. Only then will the light source reflect as a highlight in the car.