Does anyone know at what stage of the process it is too late to add something if factory ordering a terrain? I am considering adding the navigation system to my SLT-1 order but the target production week is January 18th so didn't know if it is too late to even consider (without it changing the production week 'cause I want my car . How do people that have the navigation system like it? Is it worth the extra 2k? I have a garmin and an iphone with GPS so didn't think it was necessary/worth it when I ordered but the screen really does look nice and the voice directions without a cord plugging into an outlet for power is appealing (as is the 40gb hard drive for music).