* Bulbs: The most common cause. Check all bulbs in both tail light assemblies (brake, turn signal, and tail/parking light). Even if the parking lights work, the brake light filament in the same bulb could be burnt out. Replace all bulbs as a preventative measure if they're old.
* Wiring: This is the most likely culprit if the bulbs are good.
* Look for broken or corroded wires: Carefully inspect the wiring harness leading to each tail light assembly. Look for any frayed, broken, or corroded wires, particularly where the wires enter the truck's body and near the tail light connections.
* Check the connectors: Inspect the connectors on both the wiring harness and the tail light assemblies. Corrosion can prevent a good connection. Clean them with contact cleaner and re-seat them firmly.
* Follow the wiring: Trace the brake light wire from the tail light back towards the fuse box and the brake switch (located usually near the brake pedal). A break anywhere along that circuit will prevent the brake lights from functioning. This is often difficult without a wiring diagram.
* Brake Light Switch: This switch, located near the brake pedal, tells the system to activate the brake lights when you press the pedal. If it's faulty, it won't send the signal to the tail lights, even if the CHMSL works (sometimes they have separate circuits). Testing this requires a multimeter.
* Fuse: Though less likely since the CHMSL works, check the fuse for the tail lights. (Your owner's manual will show you the fuse box diagram and which fuse controls the tail lights.)
* Grounding Issues: A poor ground connection can prevent the tail lights from working correctly. Check the ground wires connected to the tail light assemblies. Clean any corrosion and ensure a solid connection to the truck's body.
* Wiring Harness Connector: Look for a connector near the rear of the truck which may have separated or has corrosion.
Troubleshooting Steps (in order):
1. Check and replace bulbs: This is the easiest and cheapest first step.
2. Inspect wiring and connectors: This is often the source of the problem.
3. Test the brake light switch: This requires a multimeter or test light.
4. Check fuses: A blown fuse will cut power to the entire circuit.
5. Check ground connections: This is often overlooked.
6. Consult a wiring diagram: A wiring diagram for your specific year and model truck will be invaluable in tracking down the problem. You can often find these online (some sites charge for access).
If you're not comfortable working with car electrics, it's best to take your truck to a qualified mechanic. They have the tools and expertise to diagnose and repair the issue efficiently.
