I have RCA Aux inputs on my aftermarket Deck. (JVC Bongiovoni) and it has level adjustments for the inputs. I had been setting the phone vol to max and then going about twice as high as I usually do with the radio voume bu that can be a real PITA when you change from one source to another. Adjusting the line in levels helped a lot.
On a side note, I also use my Samsung Q1 Ultra UMPC with a touchscreen as my carputer. I hook it in to the same line ins and control all my media through it. I use a "front end" called Centrafuse and can do all kinds of cool stuff. One of the cool things about it is tethering my TP to my Samsung and being able to have internet access on the samsung in my car (not that I surf while I'm driving).
David