Just turn on airplane mode. That is how I got my transmitter to work. I turn on airplane mode, connect the transmitter, start playing my music and then I turn off airplane mode. I can play music and receive calls the only downside is you will get that horrible cell phone interference noise every once in a while.
Thanks a lot for the suggestion for the airplane mode, but unfortunately it doesn't work. It still charges but the sound just comes from the speakers on the iphone instead of through the car. I have an itrip auto for what it's worth. Any other suggestions?
EDIT- the tape deck adapter is a great idea except my car doesn't have it.