GMC 2008 Sierra 1500 Pickup Owner's Manual page 404

5. If the spare does not lower, turn the wrench
counterclockwise until approximately 6 inches
(15 cm) of cable is exposed.
6. Stand the wheel blocks on their shortest ends,
with the backs facing each other.
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7. Place the bottom
edge of the jack (A) on
the wheel blocks (B),
separating them so
that the jack is
balanced securely.
8. Attach the jack handle, extension, and wheel wrench
to the jack and place it (with the wheel blocks) under
the vehicle toward the front of the rear bumper.