* Single-Filament Bulb: 1964 trucks might use a single-filament bulb for both the running light and brake light. If the brake filament is burned out in the non-functioning bulb, the running light will still work. This is the simplest and cheapest fix. Check the bulbs first!
* Bad Ground: A poor ground connection at the taillight assembly on the side with the non-functioning brake light is a common issue. Corrosion can prevent proper current flow. Clean the ground wire connection and the contact point on the truck's body.
* Broken Wire in Brake Light Circuit: A break in the wire specifically in the brake light circuit for that one side is a possibility. This wire runs from the brake switch to the taillight. Inspect the wiring harness carefully for any damage, especially near the area where it enters the taillight housing.
* Faulty Brake Light Switch: The brake light switch itself might be failing. It's a relatively inexpensive part and could be malfunctioning intermittently or only providing power to one side.
* Wiring Connection at the Switch: The connection at the brake light switch could be loose, corroded, or damaged.
* Short Circuit: A short circuit in the wiring harness on the affected side could blow a fuse or prevent the brake light from working, while the running light still functions. Check your fuses.
Troubleshooting Steps:
1. Check the Bulbs: Replace both brake light bulbs as a starting point. Even if one looks good, a filament can be broken internally.
2. Inspect Wiring: Visually inspect the wiring harness from the brake light switch to both taillights for any breaks, frays, or corrosion. Pay close attention to the area near the affected taillight.
3. Test Ground Connection: Clean the ground connections at the taillights. Make sure there's good metal-to-metal contact.
4. Test the Brake Light Switch: If you have a multimeter, you can test the switch to see if it's sending power to both sides when the brake pedal is depressed. Otherwise, replacement is a good next step.
5. Check Fuses: Examine your fuse box for blown fuses related to the brake lights.
If you're not comfortable working with car electrical systems, it's best to take your truck to a qualified mechanic. Trying to troubleshoot electrical problems without the right knowledge and tools can be dangerous.
