The brief
Mr Andy's C180 had a rear lamp that refused to light up, and he'd already tried a new bulb with no change. He brought it in. When a fresh bulb doesn't fix it, the problem isn't the bulb, it's the lamp unit, the holder, the wiring, or on these the lamp's own circuitry. These rear lamp units have their bulb holders and, on some, an internal circuit board built into the assembly, and over the years the contacts corrode, the board cracks, or a connection inside fails, so even a brand-new bulb can't get a clean supply. A rear lamp unit with an internal fault isn't something you patch reliably, so it needs replacing.
The diagnosis
A check confirmed it, the bulb itself was fine, but the rear lamp unit wasn't passing a clean supply to it, the internal connection failed, which is why a new bulb still wouldn't light. The wiring back to the car and the fuse checked out, it was the lamp unit at fault. That's a lamp unit replacement, you don't reliably patch a failed internal connection, so the call was a complete rear lamp assembly, fitted and the lighting checked.
The work
The boot trim was accessed, the failed rear lamp unit removed, and a new genuine Mercedes-spec rear lamp assembly fitted, connected up, the fixings torqued and the trim refitted. The full lighting set, tail lights, brake lights, indicators, reversing lights, was run through to confirm everything worked. A check confirmed the rear lamp lit and bright, all the brake lights and indicators working, and no warning on the dash.
The outcome
The rear lamp working and bright, all the rear lighting checked and clear, and no warning on the dash. The C180 went home with the lamp sorted. A rear lamp that won't light even with a new bulb has an internal fault, so replacing the unit put it right, a real safety job rather than another bulb that wouldn't help.