Do you have a multimeter? If so, it should be relatively easy to find especially inside the car. I used one when I converted to an electric fan. If not, maybe a wiring diagram will show you a fuse to tap into. Other than that, maybe another member can chime in with a solution, Im sure someone has hardwired a radar detector or something.

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02 e53 X5 3.0i Manual Trans
13 e70 X5 xDrive35i
04 Honda S2000