Harvey JR

Hi my 56 plate A3 has an Audi Navigation Plus in dash radio sat nav unit. When I remove the fuse every is fine but as soon as I put it back in the battery drains to completely flat by the morning. Is there something in the boot I need to clean that apparently corrodes? If so where is it and how do I get to it? Or is it a case of sending the radio off for repair? It does all work correctly otherwise. Thanks.