Hello, I have a 1996 Lincoln Mark Vlll with a JBL premium sound system with a factory cell phone. To start it had a low tone thumping noise from all speakers. I took the vehicle to a reputable automotive electronics shop. I was told after listening that the problem was in the amplifier. Removed and was repaired. After reinstallation it now has a high pitch squeal that starts after it has warmed up for different lengths of time. It has now been replaced three times with similar results, a high pitch squeal. I will be really grateful for ideas. Thank you.