Very odd.
After looking at the schematics, if any signals flash, the flasher should be good. Further, if one brake lamp and/or the 3rd brake lamp functions, the brake on/off switch should be good.
That leaves it to the specific wiring or lamp socket. Since there is a common ground for the park lamp and brake/signal lamp, if the park lamp lights, the ground is good.
Against the extreme odds of a bad resistance wire or harness connector, I come back to a contact set in the infamous multifunction switch.
So I would start by testing the multi-function switch.
Since we are concentrating on the right rear circuit that would be circuit terminals 44(LB) and 5(O/LB) on the multifunction switch. With the hazard switch off and the turn switch to turn right, a good switch will indicate a closed circuit.
If you have an EVTM, it details the component testing for the multi-function switch.