Mini Add-A-Battery Kit - 65A - Blue Sea Systems
Monte.... Checkout the above link, this is what I used. Try Amazon thinking they might have it there, instructions come with the kit. I think if you look on their site it will have a schematic too. Here in Canada id guess cost wise for me just parts was on the order of $350-400. My rationale for duals is mine's a work machine and I drive in very remote areas, often alone, so getting back to the truck is a safety concern, (no cell service bla bla..). Duals are not for everyone, I also plow a bunch so it helps there too, if you can justify the cost it's a really good option. The other option is a larger battery mounted under the drivers seat. I'm saving that space for slide out drawer....
http://assets.bluesea.com/files/resources/instructions/980014350.pdf I used the Mini add a battery wiring schematic (scroll down on right) so that includes the isolator and main battery switch. Has worked flawlessly for a year now.
Sorry never did a write up on this as the schematic was pretty detailed, I used 4.0 welding cable iirc, guessing the cable is about 1.0 cm in diameter including insulation iirc, so pretty robust, also used solder on pellet ends. You heat up the ring end (propane torch) drop in pellet when it's molten plunge cable into end cup, they work great, then I slide over heat shrink tube and shrunk it on.
Cheers,
jrc