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. 

That does not happen with factory fitted equipment, so you must have an after market piece of kit that has been wired wrongly.

The power supply for the satnav needs to be taken from a circuit that is switched by the ignition key. Open the driver's side end of the dashboard to reveal the fuseboard. Using a meter, find a suitable fuse and attach your satnav power lead to it.

If the prospect of poking around your car's electrical system is a bit daunting, take it to an electrician. You could do an awful lot of damage if you are not competent to find and make electrical connections.