Where is the fuel filter located on a 1999 GMC Sierra truck?

The fuel filter location on a 1999 GMC Sierra 1500, 2500, or 3500 depends slightly on the engine. It's generally under the truck, near the fuel tank, but not directly on it. You'll likely need to get under the truck to find it.

It's usually in the frame rail, and might be somewhat difficult to access. Look for fuel lines running towards the engine and follow them; the filter is often somewhere along that line. It will be a cylindrical canister, often with some sort of connector or clamp.

Because of the variations in engine and configuration, a picture or diagram specific to your truck's VIN or engine type (e.g., 4.8L, 5.7L, 6.5L diesel, etc.) will greatly help. You can usually find this information in your owner's manual or by searching online for a diagram specific to your engine. A repair manual would also be immensely helpful.