1. Identify the Correct Fuse:
* Consult your owner's manual: This is the most reliable way to find the correct fuse for the brake lights. The manual will show a fuse box diagram.
* Check the fuse box: Your Sierra likely has two fuse boxes – one under the hood and one inside the cab (often under the dash). Look for the diagram inside the fuse box lid to identify the correct fuse.
2. Inspect the Wiring (Visual Inspection):
* Start at the brake light switch: This is usually located on the brake pedal assembly. Look for any obvious signs of damage, such as frayed wires, melted insulation, or corrosion. Pay close attention to where the wires connect to the switch.
* Trace the wires: Follow the wiring harness from the brake light switch, towards the rear of the truck. Look for any chafing against metal parts, signs of rodent damage, or areas where the insulation is compromised. This is particularly important near the chassis and around areas where the wiring is prone to movement.
* Check the brake light bulbs: While it may seem obvious, examine the brake light bulbs themselves. A shorted bulb filament can cause a short circuit. Check both the third brake light and the taillights.
* Check the wiring harness connectors: Look at each connector throughout the circuit for corrosion, loose connections or damage to the pins.
3. Test with a Multimeter (Advanced):
If the visual inspection doesn't reveal the problem, you'll need a multimeter to test the circuit. Caution: Disconnect the negative battery terminal before proceeding.
* Test the brake light switch: With the multimeter set to continuity (or ohms), check if the switch closes when you press the brake pedal.
* Test the bulbs: Check the bulbs' continuity directly – if a bulb is shorted, it will show a low resistance.
* Test for shorts in the wiring harness: This is more challenging and requires tracing the wires with the multimeter to pinpoint the location of a short. You may need to disconnect parts of the harness to isolate the problem area.
4. Possible Causes & Solutions:
* Bad Brake Light Switch: This is a common culprit. Replace the switch if it's faulty.
* Shorted Brake Light Bulb(s): Replace the faulty bulb(s).
* Damaged Wiring: Repair or replace any damaged sections of wiring. Use the correct gauge wiring and appropriately insulate any repairs.
* Corrosion in Connectors: Clean the connectors with a wire brush and electrical contact cleaner.
* Short in the Wiring Harness: This requires carefully tracing the wiring harness to find the exact location of the short. This might involve removing sections of trim and possibly some wiring.
Important Considerations:
* Safety First: Working with car electrical systems involves potential risks of electric shock. Always disconnect the negative battery terminal before starting any work.
* Professional Help: If you're not comfortable working with car electrical systems, it's best to take your truck to a qualified mechanic. Miswiring can lead to further damage or even create safety hazards.
By systematically checking these areas, you should be able to pinpoint the cause of the blown fuse and repair the short circuit in your 1994 GMC Sierra's brake light system. Remember safety first!
