Ok, well then you are stuck tracing wires.
Take off the lower light and see if the bulb holder and/or wires are corroded. There is a harness repair kit sold by Atlantic British or Roversnorth (I have one of each and, although the AB is not genuine, it is nicer) and you can buy the bulb holder as well. The wires and holder commonly corrode. However, since it is both and the third brake light connects behind the rear speaker on the passenger side I might look there first (it is more work) to see if the wiring became dislogded or cut etc. since it the two lights are on the same circuit. It is also possible that it is the ground in the same location.
PS I assume you have checked the fuses although, I am fairly sure all the brake lights are on the same one.
edit: it has occured to me that since you said door, you have a station wagon not a softtop and I wrote this with a softtop in mind and I am not 100% sure the wiring is the same as a softtop, but I assume it is similar.
*not legal advice*