Most cars today are coming with an AUX jack on the dash to feed an external audio source to the stereo like an MP3 player or Ipod etc. My 2012 Tundra's stereo has AM/FM radio, Sirius XM satellite radio, CD player with MP3 and WMA capabilities, a 1/8 inch stereo jack for aux audio, a USB jack for music on a memory stick etc and even Bluetooth wireless hookup for playing audio from a Bluetooth equipped device. I can play music from my Smartphone without physically connecting it to the stereo.What more would I ever need?
AS for providing a 1/4 inch headphone jack, lots do that now but only for rear passengers. It is illegal to operate a motor vehicle while wearing headphones.