To find it, I strongly recommend:
1. Consult your owner's manual: This is the best place to start. The manual will have a diagram of the fuse and relay boxes, clearly identifying the coolant fan relay.
2. Check the underhood fuse box: This is usually located in the engine bay, often near the battery or windshield washer fluid reservoir. The cover of the fuse box will have a diagram showing the location of each fuse and relay. Look for a relay labeled "Cooling Fan," "Radiator Fan," or something similar. The diagram might use a symbol representing a fan.
3. Look near the battery: Some Buick models put relays in a separate box or bracket near the battery. Check around this area for a relay similar to what you'd expect in the fuse box.
4. Use a multimeter (optional): If you can't find it visually, you can use a multimeter to test relays in the fuse box and near the battery to identify the one controlling the radiator fan. This requires some electrical knowledge and caution, and is only recommended if you're comfortable working with automotive electrical systems.
Without your owner's manual or a visual inspection, providing a more precise location is impossible. Remember to always disconnect the battery's negative terminal before working on electrical components in your vehicle.