I believe you are right about the all the interior lights, radio and
cigarette lighter being on the same circuit.  I would like to put all these
on the aux. battery.  I thought I would pull the fuse on this circuit, and
then  tie in to this circuit anywhere (at the radio, for instance) with a
12V feed from the aux battery.  With the correct fuse put in-line, it could
"back-feed" this whole circuit which would now be isolated from the main
battery.  This would only mean running one wire from the aux. battery to
the dash.  Does this sound OK to you?  I will double check my Bentley
tonight to see exactly what all is on this circuit, but it makes sense for
VW to have all the "hot" items on this circuit.

                                                        Jay