To be certain, you should:
1. Consult your owner's manual: This is the best resource for the specific location on your vehicle.
2. Use a repair manual: A Haynes or Chilton manual for your specific year and model will have diagrams showing the location of the oil pressure switch.
3. Look online: Search for images of a 2003 Pontiac Grand Am engine bay, specifying your engine size (2.2L or 3.4L). This can help you visually locate the switch.
Caution: Be aware that the engine will be hot if it has recently been running. Allow it to cool down completely before attempting to locate the oil pressure switch.