* Ground Fault in the Taillight Circuit: This is the most common culprit. A frayed wire in the taillight harness, especially near the body, can touch the chassis (ground). When the headlights are turned on, the increased current load can exacerbate this, causing enough current to flow through the ground fault to blow the fuse. The vibration from driving can make intermittent contact, only blowing the fuse when conditions are right.
* Short in the Headlight Circuit Affecting the Taillight Circuit: While less likely, a short in the headlight wiring could draw enough current to overload the shared circuit or pathway with the taillights, blowing the fuse. This is particularly possible if the taillights and headlights share a common ground wire or have a poorly wired junction.
* Faulty Taillight Switch: A malfunctioning taillight switch could cause a short. Though less probable than a wiring issue, it's possible the switch is drawing excessive current or causing a short when combined with the headlight circuit.
* Compromised Wiring Harness: The age of the vehicle (30+ years) means the wiring harness is likely brittle and prone to cracking and chafing. This can lead to shorts, especially in areas subject to movement (like near the hinges or under the vehicle).
* Water Ingress: Moisture in the wiring harness can cause corrosion and shorts. This is especially problematic in areas prone to water accumulation like the trunk or under the dash.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine the taillight wiring harness, both inside the vehicle and outside, looking for any signs of damage, fraying, or exposed wires. Pay close attention to areas near the body and any sharp edges. Inspect the connections at the taillights and the fuse box.
2. Check Grounds: Ensure that the ground connections for both the headlights and taillights are clean and secure. Corrosion can significantly increase resistance, causing overheating and shorts.
3. Test the Taillight Switch (if possible): If you have the electrical knowledge and tools, test the taillight switch to ensure it's not faulty.
4. Use a Test Light or Multimeter: With the headlights on, use a test light or multimeter to trace the taillight circuit. This can help you pinpoint the exact location of the short circuit. Caution: Be extremely careful when working with electricity. Disconnect the negative battery terminal before starting any electrical work.
5. Replace the Fuse with the Correct Amperage: Only replace the blown fuse with one of the *correct* amperage rating. Using a higher amperage fuse is a fire hazard.
If you're not comfortable working with car electrical systems, it's best to take your Blazer to a qualified mechanic. They have the tools and expertise to diagnose and repair the short circuit safely and efficiently.
