A little hard to see but here's how I've mounted 1315's inside the
drivers side battery box (with an Optima)

http://www.californiavan.com/battery/Battery-Pages/Image1.html

(you can only see the top of the 1315, I've written "Engine" and
"House" on the top terminals of the separator.  There was also room
to install a cig socket in the battery box that faces the rear of the
van.)

I use RED electrical tape from Home Depot for positive wires so there
is no confusion in case anyone else ever works on the van.  Don't
forget to install fuse(s) for any accessories you hook up to the Aux
battery.

>Personally, I prefer to mount the unit down inside the driver's side
>battery box, in a little recess where the seat belt wires are. This is a
>tight squeeze and requires trimming one of the mounting feet of the
>relay so the relay can go in closer to the wall. The advantage is that
>this space is otherwise useless plus all the wiring is safely tucked
>away where nothing can bump it. Models with a vented compartment under
>the driver's seat don't have quite enough room to do it this way, but
>these are rare.