GMC 1999 Sierra 1500 Pickup Owner's Manual page 132

Features & Controls
Center Floor Console
K2229
Your vehicle may have a console compartment between the bucket seats. To
open it, just squeeze the buttons on both sides of the keyhole, and pull up.
Use your round key to lock this compartment. Your console also includes a
handy place to hold cups.
PO192
Your console also has a cup holder that slides out, for the back seat
passengers to use.
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