I haven't carved a car but here's a link to a previous post about carving:
You could also start with a car pan and carve/shape it into a different car from there.
I've made a couple of cars now, you can check them out in my photos. I've found if you carve the general shape, cover in modeling chocolate first, then you can use that to shape it better, get better corners, edges, etc it holds its shape better. Then let it dry a little and then when you put the fondant on it won't round everything over too much. The big thing is make sure you get the proportions right.