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A friend of mine gave me his scanner-type LED from his car since he upgraded to a different system and LED type. Anyway, Im planning on getting some type of alarm, just not right away. But how can I rig this LED to turn on when the car is shut off and not be on while the car is running? I was thinking of mouting it high-up and wiring it similarly to the Radar Detector DIY. Would this work and would it turn on/off with the car until I get my alarm ?

 
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You would need to run the ground wire into a grounded section of your car. Then use a fuse tester and test your fuse box, with the car turned off find out which fuse still works then wire your battery into that fuse. Then wire your last wire it should be yellow into the ignition switch or use a fuse tester and find out which fuse turns off when your car is on.

 
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Set up a relay system.