some mid to highend hu will have the aux in function...
some casette player also got the function.
if u use preamp, u will need to install amp if u dun have any

alpine and pioneer(if not mistaker.) have some model support ipod..

even u connect the mp3 player line out to the bus line in, u still wont be able to turn on the function..

there is a function to turn on the aux in...but, u need to connect the bus line out to some signal to tell the hu there is something connected and the aux in will be available in the function..

conclude:
if u just connect hu line in to the mp3 line out, u will definitely wont able to hear the sound because the hu haven turn on the function..and to turn on the function, u need to make the hu to sense the device..

to some ice freak, it is cheap...because most of those hu got 24bit D/A Converter which normal hu only 1 bit

not everyone got amp ma..got preamp also no use...

this is the easiest way..but quality just like listen radio...:>

wat u need to know?