GMC 1999 Savana Van Owner's Manual page 110

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Center High-Mounted Stoplamp Your vehicle's center high mounted stoplamp is located
above the rear doors at the center of the vehicle.
If items are loaded on the roof of the vehicle, as in a
luggage carrier, care should be taken not to block or
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damage the center high mounted stoplamp unit.
If you tow a trailer that is equipped with provisions for a
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center high mounted stoplamp, see your GM dealer for
instructions on how to make it operate with your
vehicle's electrical system. As with any electrical system
modification, have the work performed by a qualified
electrical service person.
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