* Wiring Harness Issues: This is the most probable cause. Look for:
* Broken wires: Check the wiring harness running from the switch to the motor. Pay close attention to areas where the harness bends or is subject to flexing (door hinges, jambs). A broken wire is often intermittent, appearing to work sometimes and not others.
* Corroded connectors: Inspect all connectors along the harness for corrosion or loose connections. Clean and tighten them if necessary.
* Damaged switch: While you bench-tested the motor, the switch itself might be faulty and not sending the correct signal, even if it appears to be working.
* Poor ground: A poor ground connection can prevent the motor from receiving enough power. Trace the ground wire from the motor to ensure it's securely connected and clean.
* Switch Problems (despite appearing to work): Even if the switch feels fine, it could have an internal fault that only shows up under load. Try swapping the passenger-side switch with the driver's side switch (if they're similar) as a test.
* Window Regulator: The window regulator itself might be binding or jammed, preventing the motor from turning. If you can manually move the window up and down, then it's likely not the regulator. But if there's significant resistance, that's a strong possibility.
* Fuse or Circuit Breaker: Though less likely if the motor works when bench-tested, a blown fuse or tripped circuit breaker in the car's electrical system could still be affecting the power. Check the fuse box for relevant fuses. (Note that older cars might not have circuit breakers in the same way modern cars do.)
Troubleshooting Steps:
1. Visual Inspection: Carefully examine the entire wiring harness from the switch to the motor, looking for any signs of damage.
2. Continuity Test: Use a multimeter to check for continuity in the wiring harness. This will help you pinpoint any broken wires.
3. Voltage Test: With the switch activated, use a multimeter to check the voltage at the motor's connection points. If you're not getting 12V (or close to it), you've identified a problem in the wiring or switch.
4. Ground Test: Ensure a good ground connection at the motor.
5. Switch Swap: Try the driver's side switch (if applicable) to rule out a faulty switch.
Remember to disconnect the negative battery terminal before performing any electrical tests or repairs to prevent short circuits. A wiring diagram specific to your 1976 Corvette will be very helpful. You can often find these online through Corvette enthusiast forums or websites specializing in classic car parts.
