It could be a bad socket. Inspect it for signs of corrosion or excessive heat. If you are familiar with the operation or use of a test light, test to be sure current is being supplied to the socket. If it is, then check the ground wire as all electrical components must have a positive and negative circuit. I should also mention that if your vehicle is equipped with towing equipment. Check to be sure that the wires are protected, not loose, or corroded.