equalizer as the name suggests are used to equalize the various frequencies. Their main goal is that all the music frequencies should be played at the same level.
Car's interior is a very harsh environment for a sound system. U have so many things in close proximity to each other pointing in various directions. U are bound to fet reflections from different stuff that will give peak to certain frequencies and cancel some out. This is where equalizers come into play. You boost the dips and dip the.peaks to try to get a flat response.
So to answer ur question, u have to set the equalizers of ur car as per ur car's response to music. There are no fixed settings.