Chevrolet 1999 Monte Carlo Owner's Manual page 99

If you open a door, the lamps will stay on while it's open
and then turn off automatically about 18 seconds after
you close it. If you don't open a door, the lamps will
turn off after about 18 seconds, unless you press
UNLOCK or the trunk symbol on the remote keyless
entry transmitter. If you press UNLOCK or the trunk
symbol and don't open a door, the lamps will turn off
after about 55 seconds. If you press LOCK on the
remote keyless entry transmitter, the lamps will turn off
about 17 seconds after all doors are closed.
Sustained Interior Illumination includes a feature called
Theater Dimming. With Theater Dimming, the lamps
don't just turn off at the end of the delay time. Instead,
they slowly dim during the delay time until they go out.
The delay time is canceled if you turn the ignition key to
RUN or START, so the lamps will go out right away.
When the ignition is on, Sustained Interior Illumination
is inactive, which means the courtesy lamps won't
come on.
Rearview Mirror Reading Lamps
These lamps go on when you open the doors. When
the doors are closed, turn the lamps on and off with
the switches.
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