There are a million ways to splice into a wire. The easiest and cheapest is probably just using a simple crimp type butt splice. Two ends on one side one on the other. Keeps the original normal and now you have an extra for your amps... You don't really need any kind of special adapter, its just wire. If you wanted a really quick and sloppy way you could cut the stock wire, strip both halves add your output wire and just twist the three ends together and cap it off with a wire nut, or hell, even electrical tape...