Cheers mate. Been using PhotoShop in my job for years... never thought I'd use it for modifying cars though. I must say that using it in this way is much more fun then work.
In all honesty though, I'd say PhotoShop is much more about patience than skill. Play about with all the tools to see what they do, and eventually it will all come together.
Remember:
Layers are your friend - keep all the 'extra' bits on new layers and that way you can tweak them until your heart's content without damaging the original photo.