GMC 2008 Yukon Denali Owner's Manual page 112

On vehicles with a liftglass, press the button on the
underside of the license pocket applique (A) to open it.
The liftglass can also be opened by pressing the
liftglass release button on the RKE.
2-14
To open the entire liftgate, press the power liftgate
release button on the RKE or in the vehicle. See Power
Liftgate on page 2-15. You can also press the
touchpad on the underside of the liftgate handle (B).
The liftgate will open after a slight delay. The vehicle
must be in PARK (P) to open the liftgate. To close the
liftgate, use the pull cup or pull strap as an aid.
The liftgate or liftglass cannot be opened if the rear
wipers are in motion. Attempting to open the liftgate or
liftglass while the rear wipers are in motion will cause
the release of the liftglass or liftgate to delay until
the wipers are parked off the liftglass.
Both the liftglass and liftgate have an electric latch.
If the battery is disconnected or has low voltage,
the liftglass and liftgate will not open. The liftglass and
liftgate will resume operation when the battery is
reconnected and charged.
If the battery is properly connected and has adequate
voltage, and the liftgate or liftglass still will not function,
your vehicle should be taken to a dealership for service.