I have a JVC KD-R300. It came with some cables, and I don’t know what this Blue one is, here is what it says “Caution! Remote Output Only” “This is not the wire for the power supply. Please connect this wire to the remote for the amplifier, equalizer, and/or auto antenna.” I don’t have an Amplifier just in case you need that info.

Answer:
you dont need to hook it up to anything , until you get an amp. then it will run to the amps remote. turning the amp on and off with the stereo. it will also work if you have a powered antenna that goes up and down when the car turns off and on.