I know with my car if i'm using the bluetooth handsfree you can adjust the volume with the phone and my radio. Just hit the volume buttons iphone after you have connected to the bluetooth device. I have yet to find a fm transmitter that is worth a damn. I would recommend a line in if at all possible (headphone to headphone jack. Most newer cars have one or a usb hookup. Hope you get it going. Make sure your ipod app has its volume all the way up too before connecting to bluetooth then turn the bluetooth volume up.