I've just finished reinstalling the engine/trans in my 2005 Cooper S and I have one wire left that I have not found a place yet! The cable is labeled "Batt Ltg +" and "Auto Cable Aux". It is about 30 inches long with a bolt eye connector on each end. The cable is a #6 or #4 braided copper. Since it is labeled Bat +, it should be the Positive side of the battery, which leads me to the BAT + connector on the side of the air intake box. Are there TWO wires on that Bat + block? If so, one end would go there, But the other end escapes me.
Welcome to M/A! Someone with far more electrical expertise should be around to give you the answers you need...NOT me, as I am technically challenged and electrically stupid!
Doesn't sound like a factory wire....look at added options like cell phones, I pod adapters, added audio amps and such.