GMC 2002 Sierra - Manual (Page 97)

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To open a rear access door
from the inside, the front
door must be opened first.
Then, use the handle located
on the inside of the rear
access door to open it.
If your vehicle is a Crew Cab, you can open your doors
from the inside or outside. Your vehicle may also have a
feature which prevents children from opening the rear
doors. See “Child Security Locks” in the Index for
more information.
Tailgate
You can open the tailgate by lifting up on its handle
while pulling the tailgate toward you.
To shut the tailgate, firmly push it upward until
it latches.
After you put the tailgate back up, pull it back towards
you to be sure it latches securely.