How can i play Iphones navagation through the speakers?
 

I have a bluetooth modual for phone calls, but would love to play the iphones navagation through the car speakers and it would be awesom if the radio lowered when the navagatin spoke. I have Tomtoms navagation app and there is a setting on the app to lower the radio?
Could it be as easy as buying a FM transmitter that plugs in the lighter?

also there is a free navagation app for the iphone4 called Mapquest 4 mobil-its great