Chevrolet 2008 Avalanche Owner's Manual page 164

To remove each drain grate, do the following:
1. Remove the cargo panels or tonneau cover. See
Cargo Cover Panels on page 2-73 or Tonneau
Cover on page 2-91 for more information.
2. Grasp the edges of the grate and pull it out from
the vehicle. Flush the drain with clean water.
2-88
To replace the drain grate do the following:
1. Line up the clips on the vehicle with the slots in the
grate.
2. When you are sure that the clips are aligned with
the slots, push the grate down firmly.
The grate should clip into place. Do not force the grate
if it will not clip into place; realign the clips with the
holes and try again.