I have an '06 1.8S with the Rockford Fosgate system. I had the head unit replaced by a shop back in July or August. The HU is a Kenwood KDC-BT568U. I noticed a month or so ago that my rear speakers were not working. The shop told me that it was probably an issue with the factory amp. If this is the case, would I be better off replacing the amp or running the speakers directly from the HU? If this is not the issue, what could the issue be?