Rear wipers on these vans are really just decorations. I've noticed
that even on dirty wet roads and gravel roads, no crap gets onto the
rear window, and I've never actually NEEDED the wiper. Except when
dew has formed there.

Japanese vans do get dirty rear windows rapidly, so need a wiper.

It's a matter of aerodynamics... it seems that the VW doesn't have a
low-pressure zone over the window, so road-level dirty air doesn't
get that far. I don't remember my Bay or Splits getting dirty rear
windows either.