So I am going to be replacing the factory in dash 6 disc in my 04 LS sport. Its not the nav system and has the "12" speaker setup. When I pulled the radio to look at the connections, there are 4 plugs on the back of the unit. I know that one is the main connection and one is for the rear sub amp. My question is what in the world are the other two for? I suspect the larger of the other two is for the SAT or auxiliary, but the only thing I can guess for the 4th is the center channel amp. Am I correct or are they for something else?