The Mercedes-Benz C320 tail light utilizes the 12821 bulb size and the P21/5W bulb size for the rear lighting system. Data for the specific components covers the range from 2001 through 2005.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz C320 outer tail light draws from two distinct bulb families across its production years: the P21/5W and P21/4W are dual-filament ECE-standard signalling bulbs, while the 12821 is a single-filament miniature wedge bulb, making them functionally and physically incompatible with one another.
The P21/5W operates at 12V with a 21W primary filament and a 5W secondary filament, and it uses a BAY15d bayonet base. This base type features two offset pins at different heights to ensure correct polarity orientation during installation.
The P21/4W operates at 12V with a 21W primary filament and a 4W secondary filament, and it uses a BAZ15d bayonet base. The BAZ15d base differs from the BAY15d in pin offset geometry, meaning P21/4W and P21/5W bulbs are not interchangeable despite their similar appearance and wattage ratings.
The 12821 is a Philips part number designation for a T10 miniature wedge bulb with a W2.1x9.5d base, rated at 12V and 5W. It is a single-filament bulb and serves a different electrical role than the dual-filament P21-series bulbs listed alongside it.
For the Mercedes-Benz C320, the outer tail light on the 2005 model year specifies the P21/5W, which differs from the P21/4W specified for the 2001-2004 model years, indicating a base and wattage change between those production periods.
The BAY15d and BAZ15d bases used by the P21/5W and P21/4W respectively are both bayonet-style two-pin bases, but their differing pin offset configurations serve as a physical keying mechanism to prevent incorrect bulb substitution.
The 12821 wedge bulb uses a W2.1x9.5d base, which is a pinless design that relies on spring tension within the socket for retention, a fundamentally different mounting method from the bayonet bases used by the P21-series bulbs.
Both the P21/5W and P21/4W conform to ECE regulations, which are applied across European and worldwide markets, reflecting the regulatory framework under which the C320 tail lighting system was engineered.