2003 GMC Sierra 1500 Pickup Owner's Manual - Page 170

The DRL system will come on when the following conditions are met: the ignition is on, 0 Fog Lamps If your vehicle has fog lamps, use them for better vision in foggy or misty conditions. The fog lamp button is located on the left side of your instrument panel. the exterior lamps control is in AUTOj the automatic transmission is not in PARK (P), and the light sensor determines it is daytime. When the DRL are on, only your DRL lamps will be on. The taillamps, sidemarker and other lamps won't be on. The instrument panel won't be lit up either. When it begins to get dark, the automatic headlamp system will switch from DRL to the headlamps or the last chosen headlamp setting that was used. When you turn the headlamp switch off, the headlamps will go off, and your DRL lamps will illuminate, provided it is not dark outside. To idle an automatic transmission equipped vehicle with the DRL off, set the parking brake. Shifting the transmission into PARK (P) will also allow you to idle the vehicle with the DRL off. The DRL will stay off until you release the parking brake or shift the transmission out of PARK (P). To idle a manual transmission equipped vehicle with the DRL off, set the parking brake. The DRL will stay off until you release the parking brake. 3-15 Your parking lamps and/or low-beam headlamps must be on for your fog lamps to work. Press the button to turn the fog lamps on. Press the button again to turn them off. An indicator light will glow in the button when the fog lamps are on. Remember, fog lamps alone will not give off as much light as your headlamps. Never use the fog lamps in the dark without turning on the headlamps.