I would always start with the lens covers off the back bulbs so you can see the filaments.
While parked, look at the filaments and see what comes on when you try and turn. If you have a double filament bulb, the thick filament should come on and off.
If you are seeing the thin filament coming on and off, then try grounding the side of that bulb. If that fixes it, then fix the grounding where it is loose.
If that isn't the problem and you have taken off the red lenses and checked, come back and write some more.
Mike
2012 Winnebago Impulse Silver 26QP
2005 16.6 Double Eagle
2018 Jeep Wrangler JK
previously Snowbird Campers,
Triple E Motorhome and Fifth Wheel