1990 Volvo Brake/Tail Light Issues

So I have a bit of a problem. The back right tail light/brake light on my Volvo keeps going out. IT was intermittent up until a few months ago but now I can’t seem to get the light to come on at all.



I have replaced,



# All rear light bulbs (including 3rd brake and lic. plate)

# Bulb failure warning sensor (New)

# Tail Light Circuit Boards (both sides - New)



Suggestions?