I may be mistaken but there is a socket  for a fog brake light or
running light in the insert.  It looks like you only have to add  the
contact in the bottom of the socket and connect the wire to the  harness
connector to have another rear light.  I haven't done that yet but I do
have the two position switch and the extra bulb holders to make the
change.

    DrRufo
    Socal-81 vanagon