Here I am again... following up another of XcOM's excellent posts...
One really neat way to make a splice in a wire, let's say you're adding another molex for example. Slide some sleeving over the original wire, cut the insulation and expose the wire itself. Wrap the new one round it and solder. Then slide the sleeving over the joint (allow the joint to cool first, or the residual heat will start to shrink the sleeving before it's in place) but keep the wires in a Y shape.
Apply the heat and shrink the sleeving.
Once the sleeving has shrunk, but before it has cooled, squeeze the part in between the top of the Y with small pliers. Once it has cooled, remove the pliers and you should have a really neat looking splice.
This works even better with the adhesive lined sleeving.