Second horn on a 2012 RT

I have read most of the posts on beefing up the wimpy stock horn so I have a pretty goood idea of the choices. I have decided to install the dual horn set up form the earlier RT's and have already ordered the parts from my dealer. But being the electrical idiot that I am, I have a few questions about the wiring and hope someone here can help me. I purchased the 2 pin repair plug and intended to splice that into the stock horn connector wire. But I can see that it is too short to reach from where the second horn will be mounted. Can I just add some wire to the repair plug and then splice into the stock horn harness using a posi-tap? any special issues re wire gauge or method of connecting the wire extensions to the repair plug? any polarity concerns? anyhting else to worry about??
In short, if someone who has already done this can give me detailed instructions I would be most appreciative.
Thanks in advance.