The ideal would be a line input interface - probably something that emulates the factory CD changer but which just provides two phono input sockets. I trawled around Google a while ago but couldn't find anything that was compatible with the Renault hardware. I tried it again just now and also drew a blank. Maybe I'm not looking in the right places.
I gave up and bought a Belkin product that behaves similarly to the iTrip (I was using my old Creative DAP, so the iTrip wouldn't have been any use.) I specifically went for one that can be tuned across the full FM range, after previously buying a cheaper one that only offered limited options, and finding that all of the available frequencies clashed with local stations.
But even with the fully tuneable one, it quickly becomes obvious that there's not much free space left on the FM tuning scale, and what little there is will quickly disappears if you move about the country. The Belkin unit gives four pre-tuneable presets (which helps) and worked reasonably when I paired these with four presets on the radio tuner.
But there was still a fair bit of channel hopping and hunting to avoid interferrence. As long as the aerial is hooked up, it's going to happen. If I'd been keeping the car, I'd now be looking at one of the switchable, hard-wired FM modulators that intercepts the aerial input on the back of the unit. The ones I found weren't fully tuneable, but as they effectively 'unplug' the aerial when feeding in their own radio signal, conflicts shouldn't be a problem. Here's an example I found on Maplin's website.
Of course the quality from a modulator is never going to be as good as a proper line-in, but it should be at least on par with normal FM reception. It didn't really matter to me anyway, as I mostly listen to audiobooks via MP3 and stick with CD's for music. But if you use RDS for traffic and news, you will of course lose that when playing back from MP3 with the aerial effectively disconnected.