and you don't "plug" the negative wire into anything. You wrap it around a bolt like the one that holds the seat belt harness to the car just inside the door. and the negative wire is usually the black wire with a white stripe on it. If it's like some neon tubes, you're going to have to test it on a battery (your car's works fine) and touch one stipped wire to one terminal and the other to the other terminal, if it glows you know that which wire is which.