A pre wired kit should mean some sort of harness should already be installed in the rear underside, but this is often not the case. Odds are you will probably still have to run wires to battery, brake controller etc.
Wow - $400 for the trailer wiring at the dealer? And the EcoSport supposedly comes "pre-wired" for a trailer...
As nice as it is to stick with a Ford OEM solution for the trailer harness wiring, I'm tempted to just
go with a solution from U-Haul. They will run the wiring (under the car, from the battery) and
connect everything and provide a trailer connector (with diagnostic LED) and include a 2-year "no matter what"
warranty. All for about $135.