seems like I have heard that before to. Kinda up under steering column, the other flasher would be mounted in some kind of clip.If you try the hazards, do you hear anything? Sometimes the flasher will make one click only...same for signals. That would tell you if the flasher(s) were present in the circuit.
Seems like I recall later G bodies sometimes had one of the flashers in another location and did not use the second cavity in the fuse panel.