You're probably going to have to do some voltage checks around the tail lamps. I suspect you have a bad connection somewhere, but the bad connection could be anywhere between the lamp and the steering column. The first check I'd make is to check the grounds. I like to use a digital volt / ohm meter and measure the voltage drop between a known good ground and the circuit grounds while each side is powered up. You should see less than 0.01volts if the grounds are in good shape. You can also check the power feed to each lamp to see that you have equivalent voltages in both the left and right lamp circuits.
Perhaps the simplest check would be to inspect the bulbs and sockets for corrosion. Also, verify that the lamps are the same type and installed in the correct sockets. Some are dual element and others are singles. Perhaps you've got some bulbs in the wrong socket or the wrong bulb type in the socket.
