Not easy mate, ive been asking similar questions for weeks. A lot easier if you change the standard headunit as it's pretty unaccommodating to connectivity.
You can buy ridiculously expensive stuff like the connects 2 and dension ipod kits but they really don't offer much functionality compared to what you can get with an aftermarket headunit like an alpine with an ipod interface.
Can use an fm modulator to get music through your speakers but you'll have to control your ipod or other mp3 player as you always would.
Sucks really, I would happily swap my headunit but it would make the cd changer (dealer fitted) pretty useless.