how do you guys do this? I've got PS 5.0.2
First of all, you might wanna consider upgrading

, but this technique will be easily do-able in old versions of PS.
There are many ways of doing this, ranging from complex colour range selections to simple use of the eraser tool.
All I did as a quick job was to duplicate the entire image onto a new layer above the original, and remove all colour saturation. I then adjusted the contrast to bring out the blacks a little more.
Then you just zoom in and get erasing, the car only of course. This will show the colour from underneath. Then I adjusted some colour settings for the car too, but this isnt necessary.
Have a play around, its gets easier once you start doing it hands on.