Only thing I'd add to the explanation by Robert Steven Fish is that before
you run a new wire from the dashboard to the rear check the loom for a
Grey/White wire ...This is the stock wire for the rear fog lamp and VW in
their wisdom provided it in my 82 US spec van - so it could be there in
other years too.  In my case the wire was terminated in a connector block in
the engine bay and I had to run a new wire out to the lamp.  As to the
lampholder I was able to lever the holder apart and insert the spring piece
from a scrap piece - it was not to hard to do.
Reason I did the upgrade was that I shipped my US van back to the UK where a
rear fog lamp is legal requirement and gets tested each year.

Steve