For a while the radio in my 1989 M3 has squealed like crazy (the radio would make no sound until the volume knob hit a certain point, then it would create a horrendous high-pitch feedback sound- sometimes it would jump to a lower frequency.) I just replaced the factory amp based on reading I had done here, and still the same thing happens whether using the radio or a cassette tape. the wiring harness where I can see it in the trunk looks to be in decent condition. Any thoughts?