Unidentified lead in armrest

Hi to all, well I've had the Jag (2.7d sport 2006) for a week now and I'm still finding new things, latest one is in the centre arm rest section, I have a powerpoint to charge the mobile but I've also got a black lead with a jack plug, the jack plug doesn't fit an Ipod and the handbook makes no reference to it, any ideas? thanks in advance.