I am trying to create my own car kit. It is fairly easy to get the iPhone's calls to play through cars speakers (via FM transmitter or Aux input). Also, it seems that I get few complaints just having the iphone nearby and using the built in microphone. The problem I have is that when I plug in anything to the iPhone's jack, it disables the built-in microphone. Is there a cable that will not disable the built-in microphone? Alternatively, is there a cable that will allow me to split the signal so that I can send the audio into my car stereo (through my method of choice) and hook up an external microphone (to mount on my visor or wherever)?