I have a 2000 Expedition and it was prewired for a controller, even though it did not have a trailer tow package.
I bought a 7 pin trailer connector upgrade kit (Hoppy or Draw-Tite) that included the 7 pin connector for the back, fuses and relays to upgrade my Ford. All the wiring to the back of the truck was already there, but only connected to a four pin connector before I installed the upgrade. It was fairly expensive for the kit ($50), but it had all the right parts. There were GM kits also, but I did not look at them. Wal-mart sells the 7 pin connector upgrade as a separate part for about $25-30, but it does not have the fuses and relays.
I also had to buy the wire pigtail that goes from the controller to the plug under the dash in the vehicle, and I got that at U-haul. Once I wired the pigtail onto the controller, it just plugged into the factory plug under the dash.
A also remember there being an issue with the trailer package wire colors changing on certain years of the GM trucks in the early 2000's. Some folks used the old wiring diagram by mistake and caused the trailer brakes to come on all the time. Make sure you get the right color codes.
Inertia vs. timed brake controllers is always a discussion in trailer forums. I'd read through them and come to your own conclusion.