You need to plug in a fog light relay. I can send you the drawings from the 1998 S70 wiring manual (I think model year 2000 should be identical), which will clearly show where the relay goes. Send me a message at n0v8or@cox.net, and I will reply with the drawings attached as JPG files.

A standard Bosch automotive relay (available at the Radio Shack for $6) will fit (and work), but will not light the small LED in the switch that indicates when the fog lights are On. The factory relay has an extra contact specifically for that purpose.

As another post mentioned, the relay mounts in a difficult-to-access location behind the instrument panel, just forward of your left knee. I was able to plug it in without removing half the I/P, but it was done by feel; I could not see a thing.