my two cents..
I have my iPod hooked up via RCA inputs using the Blitzsafe Universal adaptor..which plugs directly into the bottom of the iPod.
The sound is about 10 db lower than playing a cd...which can be a problem when the initial source of the mp3 was an older cd..
My volume goes up to 40 on my head unit (aftermarket)
when playing a cd, I usually max out at about 27-30..
when playing the iPod, I have to crank it to 36-max..
Bottom Line...some people have more luck than others when hooking up the iPod to their car stereo