Mr. Rocket J. Squirrel said

>But it's just this slender,
>slippery thing that doesn't seem to assist with any useful function near
>as I can tell. The vermiform appendix of Vanagon interiors or something?

If the slider doesn't close (seal,seat) completely just pull "in"  - not forward - on that handle to fully close it. From the outside - just push in on the rear of the door in the same situation. 
Looking forward to your checkered table cloths in July!

Pete
80 Westy "Banana Slug"