If a customer reports that the right rear brake light is out, what is the most likely cause?

In this situation, the most likely cause of the right rear brake light being out is a bad bulb. Brake lights, like any other type of light, rely on the integrity of the bulb to function properly. Over time, bulbs can burn out due to wear and tear, leading to a complete failure of that light.

While a bad brake light switch could affect operation, it typically affects both brake lights or none at all, rather than isolating a specific side. A broken wire would usually indicate that the problem could affect multiple components or lights, not just the right rear brake light. A short in the right rear tail lamp assembly could cause multiple functions to fail or potentially blow a fuse, rather than just one brake light being out.

Given that the symptom is specifically the failure of the right rear brake light alone, a bad bulb is the most straightforward and common explanation for this kind of issue.