* Fuse: Check the fuses related to the brake lights, hazard lights, and backup lights in your fuse box. Your owner's manual will show you which fuses control these circuits. Look for blown fuses (a broken wire inside the fuse).
* Wiring: The problem could be a break or short in the main wiring harness that feeds power to all three systems. This would require tracing the wiring from the fuse box to the lights themselves. Look carefully for any damaged wires, especially near the rear of the vehicle where they are subject to movement and wear.
* Multi-function switch (turn signal switch): This switch is involved in the operation of brake lights, turn signals, and hazard lights. A faulty multi-function switch can cause all three to fail.
* Brake light bulbs: While you stated your brake lights aren't working, it's worth checking the bulbs themselves to make sure they haven't all burnt out simultaneously. This is less likely, but possible.
* Ground Connection: A poor ground connection in the rear of your truck could affect the operation of all the lights you mentioned. Check the ground wires at the rear of the vehicle for corrosion or loose connections.
* Central Tail Light Control Module (if equipped): Some vehicles have a central module that controls the tail lights. If your GMC has such a module, this could be the culprit. This is less common in 1996 vehicles, but it's worth considering if other troubleshooting steps are unsuccessful.
Troubleshooting Steps:
1. Check the fuses first. This is the easiest and cheapest thing to check.
2. Inspect the bulbs. Make sure they aren't burnt out.
3. Visually inspect the wiring harness. Look for any obvious damage, breaks, or corrosion, particularly near the rear of the truck.
4. Test the multi-function switch. This often requires some electrical testing, but a faulty switch is a common cause. A mechanic or advanced DIYer might use a multimeter to check continuity.
5. Check the ground connections. Clean any corrosion and ensure good contact.
6. If all else fails, consult a qualified mechanic or auto electrician. They have the tools and expertise to diagnose the issue accurately.
Remember to disconnect the negative battery terminal before performing any electrical work to prevent accidental shorts or shocks.
