If the plastic connector of the harness melted, this was probably caused by too much clearance between the male and female connectors of the harness and motor.....if the wires melted together, it should have blown the fuse....
When this system is in auto, the speed of the motor will vary according what you have the temp set at, and also what the actual temp of the car is.....
GM confused a lot of people, by using that 60-90 scale....people tend to think of it as a thermostat...it is in a sense, but not really with those values.....
Think about it....you set the display for 75(midrange) on auto.....in the cold of winter, do you think 75 will be warm, like your home thermostat? If you had the old slider lever in the cars of the past, would you have it in the middle on a cold winter day, or would you have it all the way to the right?
Same with auto temp....you jack that baby up to 90 in auto.....with that set at 90, it forces that blower to run at high speed all the time....when it gets too hot, manually lower the blower speed....that takes it out of auto....