To be absolutely certain:
* Consult your owner's manual: This is the best resource for the precise location and instructions.
* Check online resources: Search for "1990 Chevrolet Lumina V6 power steering fluid reservoir location" with images. You'll likely find pictures from other Lumina owners that show the reservoir's position.
Important Note: When adding power steering fluid, only use the type specified in your owner's manual. Using the wrong fluid can damage your power steering system. Also, don't overfill the reservoir.