For a normal halogen light, you don’t need to worry about polarity, but for an LED, you do. If you hook an LED up backwards, it won’t work. So, I put the digital multimeter on the plug at the Jeep end of the wire and turned the lights on. If a minus sign shows up in front of the voltage, you got it in there backwards. No minus sign means you got it right, and the wire on the red lead is positive.