I think you might struggle with a Nissan. I have the Dension ICELink in my BMW but that was factory head unit specific.
However Dension now do a universal ICE>Link FM Modulator for around £70 which will work in your X-Trail. Unlike the griffin broadcast type modulator this is actually a wired modulator. Basically you install the ipod adapter to your car and then you insert the ICE>Link between your FM Aerial and the Head Unit. I think it then has 7 preset frequencies to choose.
OK it's never going to be better than FM quality but will be a lot better than the Griffin type FM Modulator.