Taillights

remove the tail light lens or bulb socket, whichever is required to expose the bulb. Remove the bulb and turn on the headlights. Using a multimeter check for voltage at the pins inside. If voltage is there the problem will be with the bulb or contact points. If no voltage, use a thin voltage probe and back check for voltage just behind the socket by piercing the insulation on the cover. If voltage is present, the socket is bad. Replace. I've replaced many.