well, first off welcome to the Werkz. Second, any former stealership employee, as well as any former electronics store employee, can tell you that "rewire" is just a way to hit you for more $$. I've put stereoes in everything from an e30 BAUR TC to a '06 Mercedes Benz. The crossover harness is nice, but if it doesn't come made to vehicle specs, just use bullet connecters on the existing wiring to insure a replacement of the factory stereo if the time comes to part ways with the vehicle.
Process is simple. First use the female ends of the bullet connectors on the existing wiring. Then on both the factory stereo and the aftermarket headunit , use the female ends. Then install the male ends on the wiring harness of the new headunit and the wiring harness of the OEM stereo. This way, if and when the time comes to switch the new headunit to a new vehicle, it is simply plug and play.
R.I.P fast 6
Two wheels or Four, I'm happy as long as the throttle is pinned.