The Mercedes-Benz 300D brake light uses a P21W bulb size for illumination purposes. Available data covers a span from 1975 through 1993 for the specified vehicle components.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz 300D uses the P21W bulb type across all listed brake light positions, covering model years 1975-1993 for the standard brake light and 1987-1993 for the Center High Mount Stop Light.
The P21W is an ECE-standard bulb, meaning it conforms to the Economic Commission for Europe's regulations, which govern lighting equipment used in European and many worldwide automotive markets.
P21W bulbs operate on a 12-volt electrical system and carry a 21-watt power rating, which is a common output level for single-filament stop and signal applications.
The P21W uses a BA15s bayonet base, where "BA" denotes a bayonet cap fitting, "15" refers to the 15mm base width, and the lowercase "s" indicates a single contact at the base, distinguishing it from dual-contact variants.
Because the P21W is single-filament, it serves one lighting function per bulb, which is why the brake light and the Center High Mount Stop Light each use a dedicated P21W rather than a shared dual-filament unit.
The BA15s base is not interchangeable with the BA15d base, which carries two contacts. Substituting a BA15d bulb in a BA15s socket would result in an improper fit and potential electrical incompatibility.
The P21W designation is specific to the 21-watt output variant. Other bulbs sharing the BA15s base but carrying different wattage ratings, such as the P5W, are not equivalent replacements for brake light duty on the 300D as specified.