* Faulty Blower Motor Resistor: This is the most common culprit. The resistor controls the speed of the blower motor. A failing resistor can cause the motor to draw significantly more current than it should, especially on higher speeds. This extra current overload the wiring and switch.
* Short Circuit in the Blower Motor Wiring: A short circuit in the wiring harness, either within the harness itself or due to chafing against metal, can create a low-resistance path, causing excessive current flow. This can also overheat and melt the wiring and switch.
* Faulty Blower Motor: A seized or heavily worn blower motor can draw excessive current in an attempt to turn. This extra load stresses the electrical system.
* Incorrect Wiring or Repairs: Poorly executed wiring repairs or modifications can lead to increased resistance or short circuits, increasing the load on the switch and harness.
* Oversized Fuse: While seemingly counterintuitive, an oversized fuse *can* contribute to the problem. A fuse that's too large won't blow when excessive current flows, allowing the overheating to continue until the wiring melts. The fuse is there to protect the wiring, so it's crucial to use the correct amperage.
* High Amperage Draw from Accessory: An aftermarket accessory that is drawing excessive current and sharing the same circuit could also add to the load on the system.
In summary, the melting isn't a standalone issue; it's a symptom of an underlying electrical problem. Troubleshooting should focus on systematically checking each component: the blower motor itself, the resistor, the wiring harness for shorts and chafing, and ensuring the correct fuse is used. It's generally recommended to replace the resistor and carefully inspect the wiring harness as a starting point.
