1981 Westy

I was getting ready to install a new battery today (I'm rebuilding the
van... there wasnt a battery in it when I bought it) and I noticed in the
battery box behind the passenger seat the positive and negative cables like
I expected.  Also, there is a red wire coming out of a grommet that is
hooked to a small box-like thing with two terminal screws on it.  The red
wire is screwed down and on the other screw there is a red wire coming from
it.  Can anyone tell me what this is for and how I hook it up???  I thought
maybe it could be for an aux. battery but not sure.

Thanks for your help

Scott