Battery being drained by Nav System?

Hi, my X5 2005 battery is being drained each night and if I don't start the car it goes dead. Mechanic says it is the Nav system and it will cost $1200 to have it rebuilt. I am not sure if it is relaly the box or wiring. Anyone else have this issue?

Unfortunately it can be caused by the nav unit, if you want to seclude the navigation unit and be sure that it is the cause of the draw and not the wiring, then disconnect the unit and continue driving vehicle for a few days and see how it does over night. E53 X5 is VERY notorious for having Blower unit Final stage causing a draw and the AUX fan. I would start with those first as it is rare for the nav unit to be the cause, although it still very well can be the cause

I'm in the middle of debugging my E39 (same M54 engine you have) parasitic battery drain so the OP might benefit from the extensive writeup I did over here to debug and narrow it down with respect to the Final Stage Resistor (aka Final Stage Unit):
- One users experience diagnosing parasitic draw in a dead battery overnight (1)

My mechanic disconnected the nav system and the battery drain stopped. However, when he sent the Nav unit to have it rebuilt, they said there was nothing wrong with it (which I knew).
Now it is currently disconnected and as well as the entire computer screen. Besides the acutal NAV unit, could a fualty wire be the cause?

Could be the unloader relay, but that just a wild ass guess.