GMC 1999 Jimmy Owner's Manual page 67

When a new vehicle is delivered, the dealer provides
the owner with a pair of identical keys and a key
code number.
This vehicle has one
-
double sided key for the
ignition, endgate/liftgate,
-
spare tire lock (two door
vehicles) and door locks. It
will fit with either side up.
The key code number tells your dealer or a qualified
locksmith how to make extra keys. Keep this number in
a safe place. If you lose your keys, you'll be able to have
new ones made easily using this number. Your selling
dealer should also have this number.
NOTICE:
Your vehicle has a number of new features that
can help prevent theft. But you can have a lot of
trouble getting into your vehicle if you ever lock
your keys inside. You may even have to damage
your vehicle to get in. So be sure you have
extra keys.
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