[Bob Lincoln]

Bad ground on the right side, either front or rear. Probably a socket or its wiring. Those wedge base bulbs have brass clips for holders in the socket, and they are susceptible to cracking from even slight ammonia exposure over time (glass cleaner). I had a backup light socket where the clip on one side crumbled away when I pulled the bulb. That could be intermittent.

 

[John Wood]

Bad grounds were a common problem on the AP bodies light tail/brake lights. That will make the flasher not flash in many cases. A quick way to check for this problem is to take a piece of stranded hookup wire (#12 - several inches long), strip about 3/4" off each end, spread the strands flat, wedge the strands in the bulb base socket along with the bulb so that the strands only contact the side of the bulb and the side of the socket, and then connect the other end of the wire to a stud in the trunk. Usually the light housing stud provides a good body ground. Just wrap the strands around the stud clockwise and put a nut over the stud to hold it (or crimp on a tongue or loop lug). Over the years, I've fixed a few Sundance/Shadows with a non-flashing turn signal that way.

 

[ImperialCrown]

The individual socket terminals used to be replaceable and spliced in. The brass rear socket ground terminals seemed to be the worst offenders on these. A replacement tail lamp socket assembly would be the fix now.