MP3 & Transmitter

Hi guys.. I have an Altima 2.5 s with a NDSS sound system. No signs of any AUX input...or is there any hidden?
I recently bought a Fm transmitter to plug in my 12v slot to play my mp3 player but the sound isnt the same as the cd player http://www.nissanclub.com/forums/images/smilies/frown.gif
Any ways i could make this work without having to buy a whole new stereo?
Thanks a lot!

What year is it

2003