best buy has a fm transmitter that you can hard wire into the radio. it is a really nice unit i used to have one in my old honda.
As far as the cd/aux vs. cd/play button, I think they're the same thing. I think the people at PAC really don't pay that much attention to the button's label. I do know that there is a unit that does work for sale over on CGP at the store.
The FM modulators are just a tick above crap when it comes to sound quality, and the cd to cassette adapters just plain suck... (IMO)