I successfully added fog lights to my '98 S70 using the factory wiring,

Relay position #201 (upper left)
Front fog light relay. This usually comes with the "kit". If not, a standard Bosch relay will work (that's what I used), available at the Radio Shack for $6). The Bosch relay, however, is missing an extra contact that lights the LED indicator in the factory switch to verify the fog lights are on.

Relay position #213 (lower left)
The "relay" is actually a jumper to "program" fog light operation for different world markets. You could simply fabricate your own jumper with spade terminals and push it into the appropriate socket positions. It determines the power source for the relay #201 coil per the table below:

Market . . pins jumped . . power source
===========================================
Europe . . . . 2-3 . . . . parking lights
USA/Canada . . 4-3 . . . . headlights
Japan . . . . . 5-3 . . . hi beam lamps

In other words, with the USA/Canada jumper installed, fog lights will only be on when both the headlights are on AND the fog light switch is on.

In my vehicle (USA market), jumper 213 was already installed form the factory. I replaced it with my own jumper wire because I wanted fog light operation to be possible whenever the parking lights were on.