on my 850 there is a yellow connector under the dash on the driver's side just above the kick panel. if yours is similar, you need to remove the kick panel, disconnect the yellow connector (located very close to steering wheel column)that leads to your fog ligts switch and to the relay. then you have to figure uot which wire is used activate the relay to switch fog lights on. the yellow connector can be opened in 1/2s, so you then change the wire from position whre it uses "head lights ON" to position where it uses "parking lights ON" position to trigger the fog lights relay.
i would not personally re-wire it in a way that you can use it totally alone since youy may forget to turn lights off and drain the battery.
if your fog lights work w/ parking lights, they will also work w/ head lights
*** try a white wire in the yellow connector