The right knob on the radio that you use to change settings- bass, treble, fade, balance, or the radio station. Hold it down until you hear a beep. This is on the regular 6 speaker factory radio.
I was moving boxes & one held it down, when I started to go throught the settings afterwards I noticed that I could adjust midrange in addition to everything else. I just expands the sound more, but kills some of the bass. I just added more gain on my sub.