I suppose the model answer to this thread is a polite reminder that for £12 you can join CS and get access to lots of things including the guides section. I imagine this is the guide you'd be looking for...
Guide for Fitting Aux iPod cable to Standard Headunit
The gist of that thread (along with pretty pictures and a step by step guide) is that you need to buy an extra cable that plugs into the back of the headunit. You then need an RCA to 3.5mm adapter for say, an iPhone.
Alternatively if you have a quick search there are quite a few threads on devices you can buy on ebay whereby you can turn your 6 CD channels into a bunch of more modern channels, they do USB 3.5mm auxilary etc.
It may also be worth just considering an aftermarket headunit as even the cheaper (decent brand) ones have much improved sound quality over the renault headunit and also have many features such as USB, Auxilary 3.5mm, iPhone/iPod compatability, MP3 CDs etc.
EDIT: P.S. about the guide and this magical cable that you can plug into the back of the standard headunit, well there appears to be two routes you can take. One is to buy the cable which I imagine is roughly the cost of a cable... The other involves buying a little kit of cables & boxes (roughly £50) however this makes your headunit fully iPod compatible (not just "auxilary cable compatible") and means you can use the steering wheel controls to skip track, adjust volume etc which does sound very handy (speaking as someone who does not have this feature) It's also worth bearing in mind though that decent headunits (i.e. better than OEM sound quality) with iPod compatability built in seem to start at £70 or so if you can get one in a sale (Halfords usually have last years models etc in sales every now and again)
