This is from the manual:
OPERATION
The multiplexed headlamp switch has an off, parking
lamp, fog lamp and a headlamp on position. High
beams are controlled by the multiplexed multifunction
switch on the steering column. The fog lamps
are illuminated by pulling back on the headlamp
switch knob when in the parking lamp or headlamp
ON position.The headlamp switch cannot be
repaired. It must be replaced.
The Instrument Cluster monitors the headlamp
and multifunction switch operation. When the headlamp
switch is rotated to the parking lamp or On
position the Instrument Cluster sends a J1850 message
to the Front Control Module, which is mated to
the power distribution center to become the Integrated
Control Module, to illuminate the appropriate
bulbs. When the multifunction switch is activated to
the optical horn or high beam position the Instrument
Cluster illuminates the high beam indicator
and sends a J1850 message to the Front Control
Module to illuminate the appropriate bulbs.