I'm assuming that you have the factory fog lights on a NHW20 (2004-2009) Prius. In the US 2004 Owner's Manual, it is listed in Section 2-4, "Operation of Instruments and Controls,""Lights, Wipers and Defogger,""Front Fog Lights," page 107:
To turn on the fog lights, twist the band of the headlight switch lever to the position of the fog light. The fog lights come on only when the headlights are on low beam.
The band automatically returns to the ?OFF? position after you release it.
It should be listed in a similar section in your model year's Owner's Manual. There's also a little diagram showing how to turn the fog lights on.
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