I've got a 2005 sts with the 15 speaker and nav system. It was vandalized and the harness to the radio was cut and radio stolen. We've found a replacement radio pigtail at the wreckers replaced the head unit but the vocals on every audio source are very low. We've now got two head units two amps and the results with all are exactly the same, sounds like a karaoke machine. Something must be off in the wiring but we're not sure what to check, ideas anyone please?