Chevrolet 2003 Avalanche Owner's Manual page 148

2. Grasp the edges of the grate and pull it out from
the vehicle. Flush the drain with clean water.
2-74
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. Don't force the grate if
it won't clip into place; realign the clips with the holes
and try again.