The only thing I could suggest (I'm very new to this website, but I've modded the game and stalked these forums in anti-social fear for years) has already been suggested.
One of the first things I ever did in Emergency 4 was take the unmarked Crown Victoria from LA Mod and turn it into a slicktop Homeland Security car. It was generally terrible and I ended up corrupting the model on my first five tries, but I got it eventually.
Christ, that's been about six years now.
I'm not much of a modeler or a coder, but if you ever want some Photoshop advice, I'm happy to give you all the help you need.