one of the best security devices i have installed on my car and it cost me 1.97 as i dont smoke i thought why have a lighter in the car when i dont use it. so all i did was cut the main ignition wire under the dash and wire it into the cig lighter using the live on one side and the earth on the other side of the wire you have just cut. but make sure the lighter is not going to earth then go to accessory shop and buy a torpedo fitting like they have on lead lights unscrew it and solder a wire from the middle electrode to the outer which would have been the earth. push it into the lighter and you have power pull it out and it breaks the circuit. the only thing it wont do is break the lighting circuit so the only thing you can end up with is a flat battery if some one trys and steal your car and leaves the lights on. no one would ever think of this as a security device as how many cars are going around with out cig lighters in them
security for under 2 Quid 
Linear Mode
