I finally got reasonable sound to work! I am forced to use an FM Modulator as my input method and had been using the inboard M10000 audio. It ran through a long 3.5mm extension to a ground loop isolator to the FM modulator and was always a bad low signal.

I tried various cables to no avail, my iPod worked pretty well so in the end I gave up on the onboard audio and bought a USB SoundBlaster to try. I now have a USB extension cable running from the PC in to the car to the USB SB, from there to the FM Modulator. Sound is 100 times better than before and the SB only cost be $21 shipped on ebay.

I'm sure this solution isn't for everyone but it works for me so I'm thrilled.

Chris.