UPDATE: Ok, I finally got around to trying to access the underside of the roof to replace the radio antenna. I managed to remove the inside top of a cabinet and found the loose cable, but the roof hole was too far towards the rear to access.
As can be seen in an attached photo, there was no way that the antenna cable would be able to be fished out from the top of the roof. The cable end is actually attached to a disk with a stud that protrudes through the roof. There is also a ground wire attached to the stud. In my case, the stud was actually pulled loose from the disk because when I tried to twist off the antenna base on the roof, the stud was frozen in the base and I ended up twisting the stud out of the inside disk. More bad news -- the stud is soldered to the antenna cable at the disk, so the solder joint was broken.
In order to access the hole in the roof, I had to remove the wood strip (as seen in the photo in my original post). I could then pry up the ceiling enough to get my hand inside. The hole is cleverly positioned too far back to access from a cabinet, and far enough forward of the ceiling gap to make it very difficult to reach. I found that there is a metal ground plane with a hole that the stud goes through before going through the roof. In the attached photo of the ground plane you can see rust so there was obviously some minor water leakage.
I epoxied the stud back into the disk, trying to get as much contact between the stud and the broken cable as possible. I would have tried to re-solder the joint, but access in the ceiling is very difficult. Reinstalling the antenna is a two person job. One person has to hold the stud in position under the roof while the other is on the roof screwing on the base of the new antenna. Cost of the new antenna -- $10. Labor -- many hours. Not a design with maintenance in mind!
I can't see how to attach my photos so I will post this and come back with another post with the photos (hopefully).