Scosche TA03B Adaptor

Hello,
I have a 03' Celcia GT-S and am wanting to swap out the factory deck. I bought the Scosche TA03B after researching to make the swap possible I am trying to use a kenwood deck and would like to just plug it in and have use of the speakers and factory amp but I'm confised as to how to install the SLC-4 adaptor.

I currently have the Kenwood XXV-01D (also has a built in amp) I as well have the scosche harness and the blue box. From the install diagram it shows you just tighten in all the wires on both sides but states nothing about the RCA audio outputs and hooking that up, as well My deck has RCA imputs for rear and front speakers.

So my question is should i just wire in everying to the blue box and leave out the RCA jacks or should I ignore the blue box and just use the harness and connect it directly to the deck as it already has the inputs, I thinks its just more that I don't really understand what that box is doing... anyone know how exactly I should hook this deck up with the equipment?
thanx