Perhaps I've misunderstood you, but if you are using the AUX input on your car, that suggests that you are connecting from the headphone outlet on the iPod to the AUX socket on the car, in which case the iPod's menu cannot appear on the car display. If you are connected this way, select the AUX input on the car's menu and make sure that the volume on the iPod is turned up as far as you it will go.
If however, you are connected from the iPod's dock connector, to the USB input on your car, you need to use the USB input on the car's menu, not the AUX input. When using the USB connection, the volume control on the iPod should not affect the car speaker volume.
If you have managed to use the unconventional method; connecting from the dock connector to the AUX socket, use the AUX input on the car's menu, but note that the iPod menu will not be available on the car's display.
The AUX socket on the car will be a 3.5mm headphone jack:
