This was discussed in Oct 02 here is the archive URL, you might want
to search for the whole thread.   mark drillock gave a quick and dirty
way to do it:

http://gerry.vanagon.com/cgi-
bin/wa.exe?A2=ind0210A&L=vanagon&P=R5124

>  Leon wrote:
> You need to actualy rewire so that lights, radio and fridge are
> working off 2nd battery. Easy to do. Having Bentley helps.
>
> Has anyone actually written up what wires where need to be moved to
> move interior lights to the aux battery?  I've searched the archives
> twice on this and asked the list at least twice in the last year
> without finding an answer.  And Bently is NOT the easy answer unless
> you are an experienced auto electrician.  Any guidance would be
> appreciated.