[lippy]

If you already have a DIN opening in the car, get a head unit that has iPod/iPhone integration so you can use an iPod or iPhone as the source. Alternatively, most new head units have aux in 1/8" female jacks, which also would work. I would recommend against playing the music out of the headphone jack of an iPod or iPhone because it adds a small/cheap amp in the chain and won't sound as clean as playing the music through the line-out (e.g., Apple 30 pin or Lightning connector).

If you don't have a DIN opening, then do iPod/iPhone line out > RCA > amp (with line level volume attenuation) > speakers. That is what I'm doing. The JL Audio HD amps have a feature where you can control the volume of the amp using an optional pot, so the install will be clean and doesn't require a head unit.