Parasitic battery drain

[Rockstarsi]

I seem to have a parasitic battery drain on my 52 e53 3.0d. This was present when I bought it and a new battery cured the problem for about 3 weeks. Alternator is charging fine at .between 13.5-14.0v and a diagnostic check suggested maybe interior light module noted by the garage bulb checking drain. However. It appears that it may be linked to the stereo system. If I pull the first 10amp fuse in the boot which disables the radio function I have no issue starting her up the next day. This is day 2 of trying this method. Can any inform me what other function this particular fuse links to? And has anyone else had a similar problem. Driving me nuts !!