Your blower motor working even after the car is off is a good sign. This means that the car's system is preventing your car from overheating.
when my car did this it was a blown fuse
The ignition is not fused.
The 1966 Chevy Impala ignition switch can be removed by loosening the retaining ring at the top of the ignition switch. Remove the wiring harness. Reverse the process to install the new ignition switch.
faulty blower motor resistor
YES.....
Loose connection or blower motor is bad
down uner the glove box
Replace the blower motor resistor.
I owned a 75 model Impala and the blower motor was located under the hood, passenger side of vehicle,down in the fender/fender well area, hiddenbehind a/c unit
Yes
The blower motor speed resister is burned out. This part is available from your local auto parts store.
connection could be loose. Or the switch might be going out