To find it, you'll likely need to:
* Consult your owner's manual: This is the best place to start. The manual should have a diagram showing the fuse and relay locations.
* Look for a fuse box diagram: There's usually a diagram printed on the inside of the fuse box cover that shows the location of relays.
* Check online forums: Searching for "1999 Pontiac Bonneville turn signal flasher location" on automotive forums may yield helpful information and even pictures from other Bonneville owners.
Because it's under the dash, you'll probably need a flashlight and may need to remove some panels or trim pieces to access it properly. Be careful not to damage anything when accessing this area.