Im guessing this is dead now, but is there any 3D.psd skinning kits still available? I really would love that, it would make it so much easier! If somebody still has them and could upload then, that would be amazing.

In photoshop it's super simple.

Lets say you make a design, one that can be easily selected via the (Select>Load Selection). All you have to do is select it, duplicate it (Ctrl+J), then just flip it horizontally (CTRL+T, and right click>flip horizontally) and BAM you have it ready for the other side of the car. The only hard thing is, lining it up so i'ts in the same position. But really all you have to do is just move it up, and a little bit to the left/right.