Ron, The only mount you need is the one that came with the light (the triangular piece). The four stems on it penetrate the deck and then the clips attach to the stems and hold the mount in place from underneath the deck. Apparently there is nothing else that holds it. I am going to go back to the dealer tomorrow and see if I can get a print-out of the whole section. I'll let you know.
It looks like I am going to be taking out my rear seats this weekend