To be absolutely sure, I recommend:
* Consulting your owner's manual: This is the best resource for the exact location.
* Using an online repair manual specific to your vehicle: Sites like Chilton or Haynes offer detailed diagrams.
* Searching online for images of a 2003 Buick Park Avenue engine bay: Comparing your engine bay to pictures can help you identify the reservoir.
Remember to be careful when working near the engine compartment, as some parts can be hot even after the car has been off for a while.