The 'cars' look is just cut and pasting on cut out eyes and mouth from a Pixar Car, just pic the best one that suits the car. The it's a matter of blending the colors (that the hard part), play with the lighting and use the healing brush and smudge tool to make it all match.
Don;t forgot the filters too.... you can get some cool effects like this...
And with different lighting and wing, indicators and mirrors removed.... and lets not forget Lens Flare,
(you can see on the left hand side of the car where i forgot to adjust lighting layer of the bricks )
All of this took me about 10 mins....just play around with different settings, filters, colors and styles. The eyedroper and then brush tool is a good way of erasing body parts