Help! Fried Wire On Heater Fan Switch!

[too many]

Anybody have any ideas on what could have happened? A couple weeks ago I was replacing the heater controls in my '72. I ended up tearing most of the under dash housings out of the truck to lube cables and re-adjust all the vent doors. I noticed at that time , one of the wires on the back of the fan switch had been hot enough sometime to distort the plug. Everything was working great,,,til tody! I was running the AC with the fan on high, and all of a sudden noticed that "burnt wire " smell. I turned the blower switch off, but too late. Now the blower doesnt work. Any ideas what could cause my meltdown?????