I've converted several taillights...get a spare (junker) rear housing that
has the flat metal webbed connections that house the bulbs.  Take the junker
housing and pry out the metal bulb holder that is recessed in the plastic
housing.  These metal bulb holders snap in place and against the metal
webbing that runs to the wiring connector but there is no connection for the
extra hole.  I piggyback the new bulb holder onto the current taillight with
a soldier wire to the metal flat and run both on parallel.  I would not
recommend putting in a high watt bulb into this hole as it will melt.
That's why I use the taillight sized bulb which is smaller and 12W I think
to keep from burning a hole in the plastic.  I too have the dash switch you
refer to but elected to just piggyback the taillights and have more
taillight than stock.  The reflectors are very fluted and don't really give
you much more than a red glow.  I do use 50W halogen bulbs in the reverse,
brakelight and turnsignal holes as they are not constantly on and I've not
melted anything.  It's a neat trick to have the extra plastic lit up at
night, 4 vanagons in my town have commented and I've shown them how to
convert.

Good luck.

JB