As Mezo said, it has to be a connection problem to that indicator. It could be a broken wire, corroded wire or socket. Without the load of the front indicator the rear indicator will not flash. This is a balanced system and all bulbs must be connected and be the SAME wattage to flash correctly.
I was having problems with my US XT600, one side would flash but rather fast and slow, the other side would flash kind of fast and steady.
I found one bulb to be too small wattage. Once I replaced it with the same size as the others, I think 27 watts, then all flashed evenly.
I looked at the schematic and saw the speedo indicator light to be a very low voltage and was connected to the high side of the left and right indicator + voltage wire. No ground on the speedo indicator bulb, the incandescent bulb "stradled" both sides. Which ever side was NOT flashing provided the ground to the speedo bulb through the indicator bulbs. Kinda weird but it works.
Finishing touches on 84 and 86 for Colorado Trip this summer. Both running well...