A harness other than what comes with the radio is not required, but keeps you from needing to cut/splice into the factory wiring. Run the speaker out from the radio into the factory wiring and all will be well. You need to have power going to the blue wire for amp turn on. On some radios it is amp, other is it for power antenna, all the same, a wire that signals when your radio is on, and turns on something else. Get the 1784 instead of the 1786. And some different adapters use the red and yellow for opposite purposes, just follow pinouts and all is well.