There is no spare plug that I am aware of, the BMW Adventure kit uses 2 Y connectors.
One takes a signal from the headlight plug under the tank, the other takes power by a fused connection from the power socket near the starter motor
There is a good wiring diagram on the board somewhere, someone will know where, I can't find it sorry, but if someone knows where it is and post a link it should solve Dragonvalks problem.