I say this to everyone with CarPC's and noise in the audio line - ground your case. I don't mean just a ground to the PSU but actually on the outside of the metal case. If your PC case is not metal, then get one. My personal tests showed that the closer you add this ground to the audio card the better.
Make sure to get a good metal-metal connection. So sand down any paint at the grounding point(s) and I recommend the use of star washers, too. Use the same guage (or thicker) wire as you did for your 12v+ and keep it under 16" in length.
I had a problem with weird noises, clicks, hdd/cd-rom spinning, alternator/engine whine etc through the speakers for a LONG time. Finally last week I added a good ground to my case (near the sound card by the PCI slots in the back) and I'm amazed at how clean the sound is now.
I also added a line driver today and I'm VERY pleased with that as well.
