Rear Tail / Brake Light Warning

[chrispy2005]

I keep getting a message on my dash saying my rear tail / brake light is out but it's not, it's working perfectly well. It's a 2009 C200 CDI Estate. My local mechanic told me the light connectors corrode giving this warning and a conversion something or other is needed. Is this correct and if so does anyone know what part I need please? If not does anyone know what the problem could be?

Thanks in advance to anyone kind enough to help.

 

[GP801]

It senses change in resistance and will throw a warning before the bulb actually fails. Take them out to check but there will probably be some discolouration. Could also be the terminals. Whenever any of ours show a warning and the lights still work there has been evidence that a bulb is on its last legs