Iphone audio through headphone Jack

[chunter1988]

I have an aux jack in my car which allows me to play music from mp3 players such as my iphone. Although when i plug my Iphone in, i only get treb and mids no bass, sometimes no vocals. If i pull the cable out slowly i manage to get the full sound through the car but only if i hold it in that position, it sort has to be half out the jack, once i release my hand the cable pops out. I have tried different aux cables, its not the cable, the iphone works with any other device, through an aux cable just not my car. Is it something to do with mono and stereo? i have no idea ..