There is an after market Aux Battery system on my 82 Diesel.  It
was disconnected from the aux battery by the PO because he was
not sure how it worked.

I see what I assume is a relay attached to the firewall.  It is
a silver canister a little larger than a 35mm film canister.  It
has 3 bolt type connectors on the right, center, and left near
the top.  The left connector is run to the + terminal of the
primary battery.  The center connector is grounded to the frame
and runs also to the - terminal of the aux battery.  The right
connector is connected to the  + terminal of the aux battery.

The wire is not a heavy gauge.  Does this sound like it is
hooked up correctly.  Seems to me it is.  I am not sure what 12v
source energizes this relay.  Can anyone give me a better
explanation of this circuit?

Bill