your zapco amp should already come with the symbilink converter.. you want to plug that in as close as possible to your head unit with the rca it has on it. All you need is the cable that outputs from the converter to your c2k amp. They come in 6', 12' 18' foot lengths.
I have 3 sets of the 12' but i am including the extra one in a c2k amp i am selling and the other two i am going to be using. Otherwise i would sell it to you.