Originally posted by scm
What and where does the next lead come from to connect to the third point on the back of the switch??
That's the +ve supply to engine, fusebox, gadgets, etc.
Easiest way to do this is to locate the switch within reach of your original big red lead, so you simply remove this from the original (single) battery and attach it to the "output" terminal of the switch (assuming the eyelet/terminal post will fit!). Then fit red leads from the positive of each battery to the "input" posts on the switch. Finally, the negatives both go to your old black neg lead.
If you get it all in place, measure up the length of cables required and go to a motor electrical fitter to get these cables made up and soldered (unless anyone knows better - ie can you get tinned cable in this size?)