I'm not sure what the official name is for this part, but on the auto
trans console there is a strip of supposedly flexible plastic that the
shifter fits through, and that slides up and down in the selector slot
to keep the slot covered when different gears are selected.
On my van, and I assume on many others, this plastic strip had become
brittle and had therefore broken off almost completely into the console
housing.
So I went out to my local arts & crafts store, bought a piece of foam sheet
(e.g.,
http://www.michaels.com/art/online/displayProductPage?productNum=gc0864
) and cut a strip from this to replace the crumbling plastic.  Cut a
hole in the middle for the shifter to fit through, put it all back
together, and it works great!
Just thought I'd share...