GMC C series 2002 Owner's Manual page 170

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Hazard Warning Flasher Switch (If Equipped) With some options, such as U86 trailer wiring or
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RQ3 tractor application, you will have this switch.
This switch replaces the hazard switch mounted on the
steering column. It is mounted in the instrument panel,
usually to the left of and below the steering column.
Pull the switch out to make your front and rear turn
signal lamps flash on and off. A light next to the switch
will also flash. Push it back in to turn off the flashers.
The turn signal lamps won't flash if you're braking.
This switch works no matter what position the key is in,
and even if the key isn't in the ignition.
When the hazard warning flashers are on, your turn
signals won't work.
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