brake light problem

I just fixed this exact same problem yesterday on my 71 demon. I discovered a problem in the bulb socket on passenger side tail lamp.

I discovered you can push on the wires going into the socket and shove the spring and contact assembly up out of the socket to view it. Then I discovered one of the two electrode contacts was slightly corroded and touching the spring (right where the wire is soldered to the socket). When I cleaned and adjusted it, all my lights returned to normal.