What do the pins and sockets look like? If they have gotten corroded and or contaminated that might be causing some resistance.

And if you can, pull back the wire sheath and see if the wire insulation is burnt/melting.

I am going to assume your mechanic gave a cursory inspection to see if there was any external electrical issue. Did he do any troubleshooting with a meter?

An internal short would cause that sort of overload to the wiring, but I am surprised a blown fuse or code hasn't occurred.

The fact that it smokes indicates the connector is getting voltage. If the connector is getting a solid ground then it would indicate the solenoid has shorted or there is something in the connector to cause a rise in resistance