here is a good way to solder wires for those of you who are die hard solder guys. also, i recommend this for any outside connection where it can get wet or dirty, or anything to cause corrosion on copper.
i start by stripping the wires back equal lengths and then fray out the ends slightly. then i push them into each other so the strands are intertwined. pinch them closed with your fingers to make a tight bundle. if adding a wire, i do the same thing but just 2 wires on one side. once they are together heat the wire until the solder starts to absorb into it. you will use a lot more solder than you think, but you need it to fill the space between the wires and to coat the wires completely. once done add heat shrink. the heat shrink we use is a marine heatshrink so it oozes a gel type liquid and once it cools down, makes it watertight.