[joe_padavano]

The upper and lower sockets are all wired together at the bumper harness, so the switch has nothing to do with it. There isn't much to this circuit. The only options are bad bulbs, bad sockets, bad splices in the harness, or broken wires. Get a test light or VOM and just chase the circuit. Note that there are a couple of failure modes that are not obvious. First is that the terminals on the bottom of the bulb can wear down. These are usually just blobs of soft solder and vibration can wear them down to where they don't make good contact. Try installing a new bulb. The other is that the socket itself can spread to the point that the ground sleeve on the bulb makes poor contact with the socket, particularly if there is any corrosion in the socket.