Would you be best connecting it to a standard bmw 12v outlet? That way you take it with you and no power drain! Do you have on one the beak as it is? A 12v power outlet that is.
I never found s plug on my 1150, so fitted spade connectors and took power from the 'side' light bulb. Power is only live when the ignition is on or in Park, so no battery drain.