You will need to open the speaker and try to determine where the input to the amplifier is. There could be a separate board which would make that simple. If it's all on one circuit board if would require a schematic and knowledge to do. Trace the wires back from the speaker or speakers themselves to the output of the amplifier. Then try to find the input.
There will be three connections you will need to find.(R plus, L plus, and ground). A 3.55 stereo mini jack could be installed into the speaker or a cable wired directly to the board.