if you live in a city dont waste your money on an FM transmitter, I live in downtown toronto, too many signals around, not a chance for a weak FM transmitter.
Id say depending on how old the deck in your car is, go with a Tape deck to headphone jack adapter. 3.5 mm i believe *dont quote me on that
if you don't id switch up the headunit for one with ipod controls, don't know if your state/province has laws about it, but in ontario you cant use your ipod while driving, but you can navigate it using a headunit.
if you dont need the controls in your headunit. just buy a cheap one with an aux in. OR go to a shop and have them wire in to your radio, so that its a direct connection between your ipod and a certain radio station.