Very creative concept, now that you have done it you will proberly see it everywhere..
As for the shadows maybe you could try to cut out the car on its own layer (proberly is on one already hey?) then use a drow shadow effect an pull it into position from behind the preview window. (adjust opacity as required)
Once that is done and your happy, merge it onto a new empty layer so the layer effects are in effect flattened, then go and erase the shadows you don't need..
Clear as mud- just an idea
Good job all the same
Regards
Sean
