The 12N Light tester will apply a power IT WILL:- plug into the cigarette lighter, (also available with battery clips), to apply power to individual pins via a rotary switch. The rotary switch will apply a constant power source to:- Left side light(s) Left indicator Right indicator Right side light(s) Brake lights Fog lights *Reverse lights (optional extra via adapter) The unit has a red LED to indicate a power source, The whole unit is fused with a self resetting 5 amp breaker, which, if under earth fault, will trip and then reset when the power is taken away and reapplied. The red LED works with the function of the circuit breaker fuse, i.e. will go out under earth fault conditions, (earth fault referred is a short circuit that would normally blow a fuse). The unit is simple and measures 6"x5"x4" You can plug the cigar lighter fitting into a power pack to make this truly a portable low cost tester