I am getting ready to install a Kenwood KMR-M312BT in a 2004 Crossfire I just purchased. I have read the sticky about using the VW harness to wire a head unit in, but was told by Crutchfield that the Metra harness would work exactly the same, with less work. I want to retain the factory amp, and told him so, and he said the Metra would accomplish that. The more I've read on this subject, the more confused I'm becoming. Has anyone here used the Metra harness, and if so did you retain usage of the factory amp?