GMC 2001 Sierra 1500 Pickup Owner's Manual page 87

Swing-Out Windows (Extended Cab)
The latch will catch when the window is fully open and
it will also hold it in the open position.
2-4
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To open a rear swing out
window, flip the latch open
and swing the glass out.
Sliding Rear Window (If Equipped)
To open the sliding rear window, unlock the latch lever
by pushing the lever toward the passenger's side of the
vehicle. Release the lever from the latch plate and slide
the window toward the passenger's side of the vehicle.
To close the window, slide the window toward the driver's
side of the vehicle. The latch lever will automatically snap
on to the latch plate. Try to open the window without
releasing the latch lever to be sure the window is in the
locked position.