Stereo Wiring Harness Question

I have a factory am/fm stereo, but wanna buy a factory mp3 player off the internet to go in my truck. Do I need to change the wiring harness too? Or will the factory harness that came with the truck for the am/fm suffice?
Any help would be greatly appreciated.
Also, how do i get the codes for installing the radio? Dealership?

It is a straight plug and play deal EXCEPT the theft system won't allow the radio to play until it has your truck's VIN number programmed into it.