Buick 1998 Century Owner's Manual page 120

Delayed Exit Lighting
This feature illuminates your vehicle's interior for a
period of time after the ignition key is removed from
the ignition.
The vehicle's ignition must be off for Delayed Exit
Lighting to work. When the ignition key is removed,
interior illumination will activate and remain on until:
The ignition is in RUN.
The power door locks are activated.
An illumination period of 25 seconds has elapsed.
If during the illumination period a door is opened, the
timed illumination period will be canceled and the
interior lamps will remain on since a door is open.
Rearview Mirror Reading Lamps
(If Equipped)
Use the switch next to each lamp to turn them on
and off.
2-44