The current is flowing through one bulb and back feeding through another bulb, which is a direct result of having a poor ground. I would start by removing the battery cables from the battery, and then remove each bulb and clean the sockets with a plumbers 1/2" copper fitting brush. Then clean the brass part of each bulb with some fine steel wool. Next, put a light smear of Vaseline on each bulb, and reinstall them. Hook up the battery, and check to see if the problem is resolved. If it is, then there was a problem within the bulb sockets and bulbs themselves. If the problem persists, then hook a long length of 14 gauge insulated copper wire to the negative battery terminal, and with the other end of the wire with the end bare, touch it to each of the housings, rubbing it on the flat surface to get a good clean contact. Do this until you find which socket is giving you the poor ground problem. Once you figure out which lamp housing it is, then it is easier to fix. The alternative is to go around the car cleaning all the lamp sockets and lamp housing grounds, and hope that you fix it. Better to know where the problem is, and work on just the problem light. I have seen people take the lights apart a couple of times, and never fix the problem, because they never identified where the problem was located.