The 2 harnesses will allow you to aviod cutting.
Here is what you do.
Run 8 sets of speaker wires from the radio to the amp.
At the radio you want to connect the 4 wires for the amps input to the speaker wires of the harness that now plugs into your radio.
Then connect the remaing 4 wire to the harness that now connect to the factory wire harness and the other side to the amps outputs.
Connect all the power, ground, ect. connection wire to wire on the 2 harness adapters.