There's a good chance that you have all the wiring necessary, if you've already discovered the switch harness in place.
The fog light harness plugs into a plug that you should find just hangin' out adjacent to the ABS wiring near your front spindles. Once you locate that, the factory fog harness should plug right into that. Only thing remaining is to stop by your Ford dealership (or an auto parts store) and pick up the fog lamp relay that goes into the under-hood relay/fuse box on the driver's side.
2001 Ranger XLT 4.0L SOHC
Silver Frost w/ Dark Graphite interior
Summit 3" Body Lift
31x10.50 A/T tires
Explorer overhead console
Explorer auto-dim mirror
Aux1 & Aux2 inputs on factory AM/FM/CD/Cassette
Door-dinger silencer button
Seamless retractable roof and passenger-side ejection seat with steering wheel mounted controls and safety interlock