A:AnswerHello,
If you do not have an Amp or Steering wheel controls i do not believe you need this. I am not an expert but have done a number of stereos over the years. Just need ground, Live power and Ignition power then figure out which wires are for which speaker. I would imagine they make an install harness for non amp cars and they are usually pretty cheap. It makes the installation go much smoother.
A:AnswerI am not an expert, but my understanding is the harnesses connect the receiver to your factory installation of speakers, phone, etc. You can wire them manually or use the harnesses. Probably, a good investment for the harnesses.