Answer for RMD8555 (Same use for the RML8555 but it looks and installs a bit differently.)
It snaps in to the bottom door shelf - it is intended to hold bottles in place. It installs with the short flange vertical in the little gap on the front of the tray. The "fingers" point downward a bit and toward the back. Mine was a tight fit - be careful not to break the shelf.
Doug and Karen
2014 Escape 19'
2005 Toyota Tundra V8 Access Cab