If you are going to go with the FM transmitter, you get what you pay for. typically, the ones that run off your 12V outlet and are more than like 60 dollars are pretty decent
honestly i would suggest running a stereo cable to the back of your HU
this might be more than you want to spend on this, but here is alpine's newest iPod solution
http://www.alpine-usa.com/US-en/prod...X-10&view=iPod
there probably are other brands that do it for cheaper