To find it:
1. Locate the underhood fuse box: This is usually a large black plastic box located near the battery, often on the driver's side.
2. Open the fuse box: You'll likely need to either open a cover or remove screws. Consult your owner's manual for the exact procedure.
3. Check the fuse box diagram: Inside the lid or on the fuse box itself, there should be a diagram showing the location of all fuses and relays. Look for the relay designated for headlights, often labeled "Headlights," "Headlamps," or a similar term. It might also have a symbol depicting headlights.
4. Identify the relay: Headlight relays are typically larger than fuses.
If you can't find it in the underhood fuse box:
* Check the Body Control Module (BCM): Some later models may have the headlight control integrated into the BCM, which is usually located inside the cab. This is less likely for a 1998 model.
* Consult your owner's manual: Your owner's manual should have a detailed diagram of the fuse and relay boxes.
* Use a wiring diagram: A wiring diagram specific to your 1998 GMC Sierra model will precisely show the location of the relay. You can find these diagrams online (often for a fee) or at a parts store.
Remember to always disconnect the negative battery terminal before working with electrical components in your vehicle.
