Is is plug fat and yellow(fog light)? Or skinny and gray(4x4 switch)? My guess is that its just an option that is wired into the ford harness even tho your truck didnt come with that option.

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2002 XLT SuperCab 4x4 OffRoad StepSide 4.0 Auto
-Pioneer AVH-P4200DVD | Pioneer 6x8 4-Ways
-Viper 5701 Two-Way Keyless, Alarm, Start
-Mustang Dome | Expo Console | Expo Bezel & Buggman LED Mods
-Thule Roof Rack (in progress)
"I want to die young, as late as possible"