If you've got it wired like im thinking you do. Headphone to rca adapter goin into amp. The reason the volume is not what you expect is the phone has very little voltage goin out the headphone jack. I have mine wired this way and I have to really turn the volume up on the radio past what im used to. If your radio has the 3.5mm aux input you could use a male to male cord and the volume should be better then.
Its not really an amp, the headphone RCA adapter is going directly into the car's dvd player (the dvd player has the inputs), then I switch the stereo to AUX and play the music from the phone.