You'll need a ground loop insulator -- anything plugged into the 12v accessory will cause the feedback because of (I'm assuming) improper/inadequate grounding.
Something cheapo like this from Radio Shack fixed the same problem for me with my XM receiver: http://www.radtoyscentral.com/store/product.php?productid=1507670
Something cheapo like this from Radio Shack fixed the same problem for me with my XM receiver: http://www.radtoyscentral.com/store/product.php?productid=1507670