Adding rear speakers to a 2003 with cdr-23

[A_NeonTurtle]

So I've managed to pick up a rear storage cubby with speakers but am not sure exactly how to wire them to the stereo, I know people have swapped the brown plug in the back but I don't know where to find one of those, if any of ya'll have any advice or tips , I greatly appreciate it, thanks! (Also the car only has base speakers, no door speakers)