There are several ways to splice in your graph wires to the trolling motor wires. 6ga is pretty big so you're probably not going to want to use wire nuts. You could put in an inline splice and just put the smaller wire for your graph in to the crimp along with the big wire. If it were me I probably would just strip off some insulation and solder the smaller wire right onto the trolling motor wire and cover the area with electrical tape.