Mercedes-Benz CLK63 AMG front side marker bulb uses 12961 size while rear side marker bulb uses P21/5W size. Replacement chart covers 2007 to 2008 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 CLK63 AMG uses two distinct bulb types across its side marker positions for the 2007-2008 model years: the 12961 at the front and the P21/5W at the rear.
The 12961 is a Philips part number designation for a miniature wedge-style T10 bulb. It operates at 12V with a 5W output and uses a W2.1x9.5d base, which is the standard wedge pin configuration common to small automotive lighting applications.
The P21/5W follows ECE (Economic Commission for Europe) standardization, making it a globally recognized specification used across European and international markets. It operates at 12V and carries a dual-filament design rated at 21W and 5W respectively.
The dual-filament construction of the P21/5W allows a single bulb to serve two lighting functions simultaneously, with the higher 21W filament typically handling brighter signalling duties and the lower 5W filament serving a secondary role such as a position or running light.
The P21/5W uses a BAY15d bayonet base. The BAY15d designation indicates an offset double-contact bayonet fitting, where the two contact pins are positioned at different heights to prevent incorrect installation of the bulb.
The W2.1x9.5d base of the 12961 and the BAY15d base of the P21/5W are mechanically incompatible with each other. The front and rear side marker sockets on the CLK63 AMG are therefore not interchangeable.
The wattage difference between the two bulb types is substantial. The 12961 draws 5W, while the P21/5W draws up to 21W on its primary filament, reflecting the different electrical demands of the front and rear marker circuits.