Ahh is the e53 lighter a constant 12v? I thought it ran through a ACC relay so was only energised when the key was in the ignition?

If it’s a constant live than that makes it easy as he’s got the cigarette lighter one.

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E53 3.0d Sport 119,000 Miles
E61 535d Estate - Hybrid turbos and Dark-side intercooler...389hp,792nm