Hi Bob,

For an under the rear seat auxiliary battery install, I would recommend
using a dual sensing automatic charging relay like the Blue Sea Systems ACR
7610 or the Yandina Combiner 100.  The ACR monitors the voltage in both
batteries and combines the two batteries when the voltage in either battery
hits 13.2V (usually connects within 30 seconds of starting the van).  With
the ACR, there is no need for a trigger wire and it is very simple to add
solar or wind power for charging if desired.  There are tons of posts over
on www.thesamba.com about the use of ACR's and auxiliary battery systems in
general.

I have been using the Blue Seas ACR 7610 to charge two Group 27 Deep Cycle
batteries in the cabinet where my fridge used to live for years and have had
no problems with it whatsoever.  I am also running a 100W solar setup to
keep the batteries topped up while camping.

Consider pulling the power for your auxiliary battery from terminal #30 (the
post) on the starter.  This will provide a short wiring run and can be
connected easily and securely with a 5/16" ring terminal.

Jay Brown
'85 Zetec Westfalia
https://sites.google.com/site/vanagonheadlightrelays/