The right brake light does not work

The right brake light does not work. When the parking or headlights control is on, both bulbs on both left and right tail lights work. (All bulbs are working). Applying the brake normally makes both bulbs brighter (not separate bulbs for tail lights and brake lights) when tail lights are on. The top center brake light works normally. The signal is apparently not getting to the right side to light up the 2 bulbs when the tail lights are off, or to brighten them when the tail light is on. I checked fuses and pulled the tail light assembly off. Nothing abnormal in the fuses, wiring or connectors. I am thinking that if the brake switch at the pedal was the problem, no brake lights would function. I am thinking that the problem may be in the relay located in the fuse box or (hopefully not) in the wire that sends the signal to the right rear brake light. Any ideas for next step in fault trace?