For the 2003 GMC Sierra 2500HD, the low-beam headlights use 9006 bulbs, while the high-beam headlights use 9005 bulbs. The supplied fitment data does not list a headlight-type variation, so confirm the existing bulb before ordering.
Factory fog lights use 9145 LED bulbs. Front and rear turn signals use 3157A, with 3157A also listed for the front parking lights. Brake and tail lights use 3157R, while reverse lights use 3157. The center stop light takes a 921 bulb, and daytime running lights use 4114.
Other exterior bulbs include 194 for the license plate and 194NA for both front and rear side markers. Interior fitment includes a 161 ash light, 93 engine-compartment light, 194 glove-box light, and 912 trunk light. Compare the original bulb for courtesy, dome, and map lights: the listed options are 194 or 2721 for courtesy, 578 or 211-2 for dome, and 168 or 578 for map lighting.