GMC 1994 Sonoma Owner's Manual page 92

A circuit breaker protects your headlights. If you have an electrical
overload, your headlights will flicker on and off. Have your headlight
wiring checked right away if this happens.
Fog Lamps (Opfion)
Use your fog lamps
for better vision in
foggy or misty
conditions. Your
parking lights and/or
low beam headlights
must be on or your
fog lamps won't work.
The fog lamp switch
is on the instrument
panel under the
headlight switch.
Press the left side of the switch to turn the fog lamps on, and the right side
of the switch to turn them off. A light will glow in the right side of the
switch when they are on.
Remember, fog lamps alone will not give off as much light as your
headlights.
Never use your fog lamps in the dark without turning on your headlights.
Fog lamps will go off whenever your high beam headlights come on. When
the high beams go off, the fog lamps will come on again.