I was going to add a plug to the 3rd 12v battery I was adding. I just found +5 kit in the original package. Has a foldback plug that snaps into the 3rd battery plug if you are running 2 batteries. Think it cost me $10 or $12 shipped, I figured it would cost more to buy the 10 ga wire and all the plugs and do it myself.Look for +5 kit on goog, they have enough pictures to figure out the wiring if I remember right.
do not recommend the kit in any way, they mix battery types. the wiring diagram is sound however.