To incorporate an MP3 player to work with the factory installed system there is an easy and quite cheap way to do it, without any wiring, moving of units, and it is portable from vehicle to vehicle with an FM radio.
This was the solution I wanted, an mp3 player without altering in any way the in car system.
What to do..You need to purchase an FM Car Transmitter with MP3 facility and USB, not expensive, about £10-£20…….then you need a USB (I got a san disc) memory stick, I bought the 2 gig one this gives you plenty of room for as much music you can shake a stick at……I got them from the same web site …datakits.co.UK.
Then you load your music on to the memory stick put the transmitter into the cigarette lighter tune your FM radio into the frequency on the transmitter and away you go playing your music through your system as good as the rest of it.
Hope this helps.Joe