To be certain, you should:
* Consult your owner's manual: This is the best source for the precise location in your specific vehicle.
* Look for a small, usually square, black box: It might have a small, easily resettable button on it.
* Check online forums: Search for "2003 Chevy truck inertia switch location" along with your specific model (e.g., Silverado, Sierra, 1500, 2500, etc.) to see if others have posted pictures or more specific instructions.
If you can't find it after checking these resources, it's best to consult a mechanic.