Check out my post on how I connected my iPod
zerothread/230835. I am using a cassette adaptor, but you can't see it.
At first, I was not pleased with the sound of the MP3 player using a cassette adaptor. But now, I'm getting surprisingly good sound by switching the iPod's EQ to 'treble boost' and setting the car's fader two notches to the rear. Bass is just below half way and treble is two clicks away from max.
I'm getting good full range sound at all volume levels with these settings. It is also important to use the iPod's headphone out (not line level) and keep the volume of the mp3 player at ~50%. Give it a try.