1. Wiring Inspection:
* Brake Switch Connector: Carefully examine the connector at the brake switch itself. Make sure it's securely plugged in and that none of the pins are bent or corroded. A poor connection here is a common culprit.
* Wiring Harness: Trace the wiring harness from the brake switch to the tail lights. Look for any visible damage, breaks, or chafing in the wires. Pay close attention to areas where the wires might rub against metal parts of the vehicle. A broken wire is a likely suspect.
* Continuity Test (Requires a multimeter): This is the most reliable way to check for breaks. With the ignition off, disconnect the brake switch connector. Using your multimeter, check for continuity (a closed circuit) between the brake light wire at the switch and the corresponding wire at the tail light connector. If you don't have continuity, you've found a break in the wiring. Repeat this for both brake lights.
* Follow the Wiring Diagram: A wiring diagram for your specific 1990 Chevy 1500 will be invaluable. It will show you the exact path of the wires and allow you to pinpoint the location of any potential problems. You can usually find these online through repair manuals or automotive forums.
2. Ground Connection:
* Tail Light Ground: A poor ground connection is a very common reason for brake light failure. The ground wire for the tail lights might be corroded, loose, or broken. Locate the ground wire for the tail lights (it's usually a black wire) and check its connection to the vehicle's body. Clean any corrosion and ensure a solid connection.
* Brake Switch Ground: While less common, ensure the brake switch itself has a good ground. This involves tracing the ground wire from the switch and ensuring a clean connection to the vehicle chassis.
3. Other Potential Issues (Less Likely):
* Brake Light Fuse (Double-Check): Although you say you checked the fuses, double-check to ensure you checked the *correct* fuses for the brake lights. Sometimes there might be multiple fuses involved in the circuit, or a fuse that appears good might have an internal break.
* Wiring Harness Connector: Check all connectors in the wiring harness related to the brake lights for corrosion or loose connections.
* Turn Signal Switch (Rare): In some rare cases, a faulty turn signal switch can interfere with the brake light circuit.
* Body Control Module (BCM): On later model vehicles, a BCM might control brake lights. This is less likely on a 1990 model, but it's worth noting.
Troubleshooting Tips:
* Have a helper: It's easier to test brake lights if someone can operate the brake pedal while you check the lights and wiring.
* Safety First: Disconnect the negative battery terminal before working on any electrical components.
* Use a test light: A test light can help you visually confirm if there is power at certain points in the circuit.
If you've checked all of these and still can't find the problem, you might need to consult a qualified mechanic. They have specialized tools and expertise that can help diagnose more complex electrical issues. Remember to be methodical and patient; troubleshooting electrical problems often requires a systematic approach.
