For the 2004 GMC Sierra 2500HD, the low-beam headlights use 9006 bulbs, while the high-beam headlights use 9005 bulbs. The fog lights are listed for 9145 LED bulbs. No headlight-type variation is specified in the supplied fitment data.
Other useful exterior fitments include 4157NA bulbs for the front turn signals and parking lights, 3157R bulbs for the brake and tail lights, and a 3157A bulb for the rear turn signals. Reverse lights use 3157 bulbs, the center stop light uses a 921 bulb, and daytime running lights use 4114 bulbs. License plate lights take 194 bulbs, while front and rear side markers use 194NA bulbs.
Interior fitments include 194 for the glove box and 578 for the map light. The courtesy light may use either 194 or 2721, and the dome light may use either 578 or 211-2; compare the original bulb before ordering. The trunk light uses 912.