1. Verify the Switch (But Don't Assume It's the Only Problem):
* Test the Switch: Disconnect the switch from the wiring harness. With a multimeter, check for continuity between the terminals when the brake pedal is depressed and released. It should only show continuity when the pedal is pressed. If it's always showing continuity, the switch is bad. However, a bad switch *doesn't explain why the light stays on after you disconnect the battery*.
2. Investigate the Wiring:
This is the most likely source of the problem. The brake light circuit in your truck likely includes:
* Brake Pedal Switch: (Your Kmart switch)
* Wiring Harness: This is where shorts are likely.
* Fuse: Check the fuse related to the brake lights. A blown fuse points to a short somewhere.
* Brake Light Bulbs: A short in the bulb socket can cause this.
* Tail Light Assemblies: Check for any shorting in the tail light assemblies themselves.
Troubleshooting Steps for the Wiring:
* Visual Inspection: Carefully examine all wiring related to the brake lights, looking for any chafing, bare wires, or damaged insulation. Pay special attention to areas where the wires might rub against metal parts. Look at the wiring harness running from the switch to the taillights.
* Continuity Test: With the switch disconnected, use your multimeter to check for continuity between the brake light wire and ground. There should be *no* continuity. If you find continuity, you've located a short circuit. Trace the wire back from the point of the short to find the cause.
* Fuse Check: Again, check the appropriate fuse. If it's blown, replace it (with the correct amperage fuse). If it blows again immediately, you have a definite short circuit somewhere in the system.
* Bulb Check: Check the brake light bulbs themselves and their sockets for any damage or shorts. Sometimes a faulty bulb can create a strange circuit.
* Ground Check: Ensure a good ground connection exists for your brake lights. A poor ground can cause all sorts of strange electrical behavior.
3. Why Disconnecting the Battery Doesn't Solve it (Sometimes):
Sometimes, even after disconnecting the battery, a parasitic draw can persist. This means a small current is still flowing somewhere in the system, keeping the light on even without power. The short is likely the culprit here.
In summary: While the Kmart switch *could* be defective, the more likely culprit is a short circuit somewhere in the wiring harness related to your brake lights. Systematic checking of the wiring is crucial to finding and fixing the problem. A wiring diagram for your specific 1965 truck would be extremely helpful in this process. You can find these online through various automotive resource sites.
