Mercedes-Benz CLK350 front side marker bulbs use 12961 size while rear side marker bulbs use P21/5W size. The replacement chart covers 2006 through 2009 vehicle production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz CLK350 uses two distinct bulb types across its front and rear side marker positions for the 2006-2009 model years.
The front side marker across all four model years calls for the 12961, a designation originating as a Philips part number for a T10 miniature wedge bulb with a W2.1x9.5d base, rated at 12V and 5W.
The W2.1x9.5d base is a wedge-style connector, meaning the bulb is held in place by tension from the socket rather than a locking bayonet mechanism.
The rear side marker across the same 2006-2009 span uses the P21/5W, an ECE-standard dual-filament bulb rated at 12V with a 21W primary filament and a 5W secondary filament.
The P21/5W uses a BAY15d bayonet base, which features offset pins to prevent incorrect installation of the dual-filament bulb in a single-filament socket.
Because the front and rear side markers use physically incompatible base types, a W2.1x9.5d wedge bulb and a BAY15d bayonet bulb are not interchangeable between positions.
The wattage difference between the two bulb types is substantial, with the P21/5W rear bulb drawing up to 21W on its primary filament compared to the 5W draw of the front 12961 bulb.
The P21/5W designation follows ECE (Economic Commission for Europe) nomenclature, a regulatory framework used across European and many worldwide markets for vehicle lighting standardization.