Originally Posted by
GraphiteGhost

While your in the bowels of the lower trunk, slice an X in the center of each of the two well plugs leading out the bottom of the trunk. There is one on each side. The X will allow water to drain but the rubber has enough stiffness to keep debris from entering the lower bay area. The water in the bottom is especially expensive if it destroys the central locking module on the lower right corner of the right bay below the Styrofoam insert below the flip-up carpet. Before you reassemble, remove and inspect/dry out any moisture (hoping no water made it into it). Gook luck! Oh, pay attention to those who recommend using something that is easily removed, as you never know if you'll be removing that assembly at some time in the future.

The Mercedes Forum had a post recommending that the pump be placed in a nice plastic bag before buttoning everything down.
Makes a lot of sense to me.
Anther post said to put the pump in upside down.