Aux heating BMW X5 3,0i 2001

Hi,
Last year I had a Webasto Thermo Top P installed on my X5 3,0 (2001). I'm having a bit of trouble with it though. It seems as though the fan/ventilation system is using an awful lot of amperage (12-15 amps) when the aux heater is running, resulting very quickly in a drained battery. IMy mechanic have tested a Toyota with the same aux heater, and on this car the fan system only pulls around 2 amps. BIG DIFFERENCE!!

Does anyone have any experience with aux heaters? Does anyone know where I can pull a correct wiring diagram for this kind?

I know of examples where X5's run with aux heaters, and I can't imagine anyone being satisfied with this kind of trouble. Running out of battery every time the heaters has been turned on more than half an hour!!

I hope to get lots of helps on this Thank you all very much..!