hey Isamu,
i plug my Imod right into the back of my Alpine car cd receiver...it works amazingly well.
There aren't any on-screen instructions, i still have to look at the Imod to work it...but the sound quality is fantastic.
Mine is a $200 base model & required a $10 adapter cable from Crutchfiled to accept the RCA inputs. from the back of the receiver to my Imod is just a normal 3.5mm mini plug (1/4") to RCA male plug cable.
I've looked at the newer interfaces for cars, i don't see how they can do good things for the sound quality. the interfaces are really so you can control Ipod through the stereo. For me, the Imod sits about 4" from my hand, so hitting the NEXT button isn't putting me or others in danger - hopefully!
Imod in the car is awesome.