Meka..the "50's coupe" is pretty nice, especially if it was done in 15 min.
I can see that you need to clean up the edges around the car, this is easy to do with the eraser, if the cars is in its own layer.
another trick to getting the roof to match after you chop it, is to use "scale" in the transform menu.
once you have the car cut out and trimmed. you can select it. then start a new layer, then fill that layer with a solid color. to get a realistic look, use OVERLAY for that layer. then erase the overlapping color around windows, grilles, etc...