Use the Wand Tool, (W), to select the area representing the atmosphere and the, (G), tool to make the closest areas darker.
Make the Outline Layer current and use the Wand Tool, (W), to select the outside of the car as shown. Inverse the selection (Select > Inverse, or ctrl + shift + i). Use the Pen Tool, (P), and draw a path like the one shown.
Select the Brush Tool, (B), and choose a thick brush with faded edges.
Select the Pen Tool, (P), right click the path and choose stroke path from the options making sure the brush tool is selected as the Tool.
Press esc twice followed by Select > Deselect, or ctrl D to deselect everything
Continue to use the described tools above to refine your car adding highlights and shadows.
The shut lines are where two panels meet. To create highlights for these activate your original paths by selecting them from the Paths palette (Window > Paths) and stroking these in white in a new layer as oppose to black as described before. Then nudge this layer to the right and down a little using the arrow keys