Hey,
the problem was that your shadow was hard, and completely under the car.
There's like 4 different light sources, so there's 5 different shadows.
1 Under the car
2 going towards us
3 Going left
4 Going right, which is lighter cuz the light source is further away.
5 the contact shadows of the wheels
Then also the car is lit from behind, so there's a certain ammount of lightbloom it should have. Light leaking from behind creating lightening on/around the car.
I just drew 5 shadows on 5 layers, and did them at different opacity.
Then I did a selection of white, subtracted the car from it, but feathered that selection heavily, so it went blurry white on the car. Then deleted the leftover white by subtracting everything but the car from the white.
And I made the windows a little see through (20% black on a mask)
http://i25.photobucket.com/albums/c9...l/helpcopy.jpg
Hope that helps