The Mercedes-Benz CLK500 front side marker bulb uses size 12961, while the rear side marker bulb uses sizes P21/5W and 12961. The replacement chart contains vehicle production years and bulb size specifications covering 2003 to 2006.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 12961 is a Philips part number designating a T10 miniature wedge bulb with a W2.1x9.5d base, rated at 12V and 5W. The T10 form factor is one of the most widely used in automotive marker and courtesy lighting applications.
The W2.1x9.5d base designation refers to a wedge-style connection where the bulb's glass envelope itself acts as the contact, eliminating a separate metal base. This base type is standard across a broad range of low-wattage automotive bulbs.
The Mercedes-Benz CLK500 uses the 12961 bulb for the front side marker across all four model years covered in the specifications, specifically 2003-2006.
The rear side marker of the CLK500 also uses the 12961 bulb for the 2003-2005 model years.
For the 2006 model year, the CLK500 rear side marker transitions to the P21/5W bulb, which differs substantially from the 12961 in both construction and wattage.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V with two filaments drawing 21W and 5W respectively. It uses a BAY15d bayonet base, which features two offset pins to ensure correct polarity orientation during installation.
The BAY15d base of the P21/5W is physically incompatible with the W2.1x9.5d wedge socket used by the 12961. These two bulb types are not interchangeable, and the socket design differs between the 2006 rear side marker and all other positions using the 12961.
The P21/5W carries a significantly higher primary filament wattage of 21W compared to the 5W rating of the 12961, reflecting its role in applications that require a brighter output, such as combined stop and tail or signalling functions.
The ECE designation on the P21/5W indicates compliance with United Nations Economic Commission for Europe lighting regulations, a standard widely adopted outside North America for automotive signal bulbs.
Because the 2006 CLK500 rear side marker uses a P21/5W while the front side marker retains the 12961, the front and rear positions on that model year require two distinct bulb types with different bases, wattages, and filament configurations.