I started by tracing the silhouette of the car (pic 3). I always kept one invisible copy of that silhouette to copy later for masks and pathfinder operations such as intersection. This will make sure that all objects stay inside that border and no shadow, reflection, highlight or whatever goes beyond or not far enough. When using the pen tool and you pull the bezeir handles you might realize that you should have placed the vertice somewhere else. No problem, while keeping the mouse pressed, hold the space-bar to replace the vertex.