Ed, that should be a small trim cover that goes over the end of the power seat track. It just slides on, and covers the area where the end of the track is exposed.
Ed and Eddie, I could tell by the crash book it was something for the seat. It doesn't show those parts. Eddie, if you can't figure out where it goes, I will show you when I get there.
I sure hope the new interior isnt white....
Not sure about the plastic piece though. The only one I know of is on the tracks and they are about 15" or so long. Maybe a clip for the tubes or wiring under the seat?