My dads 2000 x5 has been having a battery drain also. We changed batteries, alternator, changed the Headunit, disconnected various components / pulled fuses trying to figure out where the draw was coming from. Dealership had it for 2 weeks and charged 700 for diagnosing saying it was the instrument cluster. Replaced that. Still killing the battery. Finally took it to a private guy that does independent diagnosing for dealerships in the area. He had it for a 4 hours and was able to get it fixed:

"Memory wire" on the head unit
Rear defrost
Headlight switch
And rear drivers side tail light

all drawing a current when the vehicle was "asleep". Not sure exactly what he did to fix the issues because his explaination was very technical. But my advice is to find someone that specializes in electrical. Not all dealers / Indies are able to handle these issues without throwing money at it. The last guy charged $400 and it was fixed 100%

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2000 E53 4.4
2011 E70 35d