The A/C control unit in my 2000 A4 TDI locks when I press the button to switch on the heated rear screen - no matter which button or switch I then try, nothing works. The only way to 'unlock' it is to disconnect the battery. I'm told I need a complete new control unit/display, part no. 8D0 820 043M B98 (I'm not sure what the B98 means). There's a control unit on ebay right now but without the B98 suffix; is it the right one?