* Overheating Wiring Harness: The most probable cause is a section of the wiring harness, possibly near the HVAC controls or fuse box, that's overheating and melting the insulation. Running the system at high settings increases current draw, making this more likely. The intermittent nature of the problem before complete failure supports this. Look closely at the wiring harness in the area of the HVAC controls and the fuse box under the dash – check for melted insulation, burnt wires, or any signs of damage.
* Fuse or Circuit Breaker (Though less likely): While a blown fuse or tripped circuit breaker *could* affect either the blower or windows, the fact that *both* failed simultaneously makes this less probable. Still, check your fuses and any inline circuit breakers (if present) related to the HVAC system and power windows.
* Ignition Switch: A failing ignition switch can sometimes cause intermittent or complete power failures to accessories. While less likely than the wiring harness, it's worth considering if you're experiencing other electrical gremlins.
* Bad Ground: A poor ground connection somewhere in the circuit for either the blower or windows (or a shared ground) can lead to overheating and failure. This would be more likely to cause intermittent problems *before* complete failure. Check for corrosion on ground straps and connections.
* Body Control Module (BCM) or related relay: While less common, a problem with the BCM (if your truck has one), or a relay controlling power to the blower motor and/or power windows, could be the cause. This is more likely if other electrical systems are also malfunctioning.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine the wiring harnesses in the areas mentioned above. Look for any signs of melting, burning, or loose connections. Pay extra attention to areas where wires might rub against metal or other components.
2. Fuse and Circuit Breaker Check: Thoroughly check all fuses and circuit breakers related to the HVAC system and power windows. Replace any blown fuses, but be aware that repeatedly blowing the same fuse points to a deeper underlying problem.
3. Ground Connection Check: Locate the ground points for the blower motor and power windows and clean them thoroughly with a wire brush. Apply dielectric grease to prevent future corrosion.
4. Professional Help: If you're not comfortable working with car electrical systems, it's best to take your truck to a qualified mechanic. Diagnosing electrical problems can be challenging, and incorrect repairs can lead to further damage.
The wiring harness is the most likely culprit given the symptoms. Finding the damaged section might require carefully tracing the wires from the blower motor and power window switches back towards the fuse box and HVAC controls. A multimeter can help pinpoint breaks in the circuit. Remember safety first – disconnect the battery negative terminal before working on any electrical components.
