Contact the car manufacturer. Find out if it is compatible with your model iPod Nono. You must be specific as which model you have in order to receive the correct information from the car manufacturer.
Also check in settings 'Sound check' try changing it.
I dont think the Volume Limit in settings can cause this when connected to dock connector, but its easy to try to turn it to max. If that doesnt work try it on another car, or take it to Apple for free check.