I am not sure about what you mean by "master switching light".
If it is the interior lights.....and they go off after you get moving at 20mph.....then you have a dirty or defective door sensor switch........if you have verified that all the doors are closed.
On the driver and passenger front doors, the sensor is in the latch mechanism........the part that holds the door closed.......the switch is located inside the door......next to the latch.