Chevrolet 2004 Avalanche Owner's Manual page 146

Left Side Shown, Right Side Similar
Push the panel forward until it is snug against the
other panel and then let the back of the panel down
being sure that the pegs align with the receivers.
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7. Push the latches away from you, starting with the
left latch, to lock the panel in place. You should
hear a click when each latch locks correctly.