I just did this with the same confusion ,, Some of the Base model Mustangs did not have any wires in the Number 3 and 6 pin on the back of the headlight switch , Those that do Just Cut the existing wires and Connect the ones from the Fog light harrness , Orange to Orange/green , Purple to Purple/ Orange If you have the terminal tool just remove and replace , The Purple wire is to turn on the light in the switch and the orange is to turn on the relay that will turn on the Fog Lights , ,, The Yellow and white wire did not exsist in my Harness , it is hooked up to the High Beam cut out relay , You can turn on your bright lights and find what wire turns on your high beams at the Light Socket ,, I think it was white/green,, you dont hook up the wire your fogs will work on both Low/high beam ,, The Red power wires and grounds were explained in the directions
