To be certain, you should:
* Consult your vehicle's repair manual: This is the most reliable source for the precise location.
* Use a repair manual diagram: Many online resources, like Haynes or Chilton manuals, offer diagrams showing sensor locations.
* Search online for a diagram specific to your Bonneville's engine: Search using terms like "1999 Pontiac Bonneville [3.8L or 3.1L] crankshaft position sensor location diagram".
Visually inspecting the area described above with a diagram in hand will greatly increase your chances of finding it. Remember to be careful when working around engine components.