Its called a Line Output Converter... it takes speaker level inputs and gives you a line level signal... that the amps RCA's can take. If the amp already has these "high level" inputs, you dont need the adapter. Just tap into the wirring at the rear speakers and run the wires to the amp or LOC.
The down side of that setup is that the stock deck for that car does not have a remote turn on output... so there is no way to trigger the amp when you turn the deck on... you can wire it so its on when the key is on... but you tend to get feedback from the rear speakers when the deck is off. I'd say wire the amp to a switch so you can shut it down when you have your deck off.