Cig lighter is 12V switched. It only works when the car is on. You may also be able to use a wire in the back of the seat that is switched. When you get the back off the seat there should be wires running to the headrest or the seat fans (if you have ventalated seats and/or comfort seats). With the aux/cigarette lighter there will be three wires 1) constant 2) switched 3) ground. Use a voltmeter to determine which one is switched. Test each wire while the car is off then when on. Use the 3M t-taps to tap your wire, no cutting thus not voiding your warranty.

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2013 BMW X6M-current
2013 Audi S4-current
2007 BMW X5 4.8i (E70)-sold