Hi there,
Are you using a cable into a 3.5mm socket on the front of the unit?
If you are you should be able to control your iPhone settings from your iPhone. Knock your EQ off and turn the volume to about half way on the iPhone itself. This is effectively a master volume and you should then adjust your car unit's volume to your preference. You may have to play with balancing iPhone (master) volume and the car volume to get the best results.
If you connect iPhone into your car unit by a rear aux input (into the glovebox for example) with the standard 30 pin iPod/iPhone connector, I am mystified because you have no control over the iPhone by using it's interface and it may point to a problem with your in-car unit. iPhone is controlled by the unit into which it is plugged and the screen of iPhone tends to say something like 'iPod accessory' and fails to resond to any touching. I use an Alpine unit which allows full functionality allowing the same sort of searches as you'd get from operating the iPhone direct eg searching by song, artist, album, genre etc.
I hope this asists you
regards
The Screw