Illl assume its not dim, at night, a key check, in all cases like this.
the lamp is bad. (used lamps can have a shorted base so only NEW lamps work best)
the socket is bad. (corroded
the wires are cut by trailer hitch goons, (wire hackers (look and see)
its not the fuse or the switch, the problem from the harness at left rear of right rear.
the signal harness usually feeds left then right side.
so the wire from there to there can in fact fail, or be collision damage
or trailer hitch hacked.. right there.