Hi John,
It is possible that the power supply for the BMW mount isn’t connected correctly. In principle, the N3 should switch on and off in sync with the BMW’s ignition. Please check that the existing power cable is properly connected. If you wait a little longer after turning off the ignition, does the N3 switch off then? With BMWs, certain switched wires may not lose power until 30 to 60 seconds after the ignition is turned off. Consequently, it might take a bit longer for the N3 to shut down after the ignition is switched off.
Also, is the N3 seated correctly in the mount so that all the electrical contacts on the back are making a proper connection?