Non nav GTRs have no way of inputting directly to the AV system other than by using an FM modulator.
This splices into the FM aerial on the lower AV unit and allows iPhone to play through a frequency on the radio. It is hardwired so doesnt suffer interference and the sound is very good.
I have the Dension system installed in my GTR and I wrote about it almost 3 years ago here
Sound is pretty good and allows phone to function as normal, music returns after a call.