The trick is to make it black but still have glossy reflections - otherwise you end up with a dark flat car - or a gray car.
(I didn't have a white car pic handy but this very light silver should demonstrate it.)
1. Mask the metalwork
2. Add a curves layer with a fairly extreme curve and a Hue saturation layer. These should darken and desaturate the car but will take out many of the reflections so it will look flat




3. Duplicate those top three layers . Go into curves and really mangle the curve to bring out the reflections and shape.
This will look unnatural - but don't worry you are just trying to bring out the shapes with highlights



4. Now change the top (mangled) car layer blending mode to screen and reduce the opacity. You are aiming to make those highs blend naturally

Result :
From this :

To this :

I hope that points you along the way
Dave