Ground loop isolator may do the job, it fits somewhere bewteen head unit and ampid you have one, also know as noise supressors. I had one of the units shin is referring to when I ran my carPC - it was about $10 shipped from Hong Kong and it worked a treat.
I am learning that the Iphone and Itouch are not the best with these interfaces. I'm unsure whether I'm going to run another GB or not at this stage. If I do it will be for this forum only, not OMC as well.