Chevrolet 2004 Avalanche Owner's Manual page 154

Removing the Tonneau Cover
1. Starting at the rear corner of the driver's side of the
vehicle, pull the loop (F) toward you and then pull
up so the plastic lip (E) releases from the channel in
the rear rail (D) and the side rail (C).
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2. Pull the back of the cover out from the rear rail (D).