The black plastic aero piece that is under the sill in front of the rear driver side. The bottom rubber piece was hanging loose. It is attached to the upper plastic portion by 4 rubber force fit plugs tapered in shape so once they are pushed in they cannot come out unless something hits them and tears them loose. Piece is held on by 4 screws, phillips head, and was easily exchanged. I had to jack up the car and remove the wheel in order to get at it properly as two nuts had to be removed and the splash plastic panel in the front part of the wheel well taken out in order to remove a hidden mounting screw. Took about a half hour and I didn't break anything...

... This was quoted as a $160 dollar job by the dealer. I bought the piece for $44 plus postage. Now the other side looks shabby since it has been sand blasted for 43k miles so I may have to replace it too.
AKL