Fitted a new (to me!) Concert HU today and it won't link to the CD changer in the back. The old one worked fine. The changer is a sony unit, so running a differant BUS lead and adaptor - checked this lead and all looks ok. Ruled out the HU as I tried another Concert HU and that has the same problem in my car but conversely my HU works in his car - he does, however, have an original Audi CD changer. When you plug in the plug/lead for the CD, it makes a single click noise, but that's it - no power to the unit at all. Also, occasionally when you press mode key, the unit displays 'NO CD' any ideas....please?!