The cable one the very left is the negative cable connected by the fuel pump. That is a very bad location for that negative cable in case of a fuel leak. The cable that has a metal tie down strap center of picture is positive cable. But just to make sure crawl under truck and trace that wire to starter. If you have some red electrical tape just mark the two ends of positive cable for future reference. I would also move that negative cable to top of alternator bracket or almost anywhere else. Once again trace the wire with the hold down on frame to starter to make sure that is the positive cable
