I have the above stereo in my car with the Bose sound system option and six disk changer. The stereo is playing up, sometimes it will go to max volume or no volume as soon as you touch the volume control, regardless which direction it is turned. I believe this is quite a common fault. Do you know will any Audi concert stereo tape/tuner fit, is there only one type? If there are many types what do I need to look for?