Here is an easy way to check to see if the entire header switch system is functional. Unplug the connector at the rotary brush assembly on the electric clutch and plug it into the service lights under the gull wing door. The donnector is located behing the door outside the cab above the black central electronics box. Plug the harness into the male end at the electric clutch and then turn on the header switch. OF cource you have to be sitting in the seat. If the lights turn on it is the rotary brush assembly. If not then it could either be the header switch, the seat switch, armrest control unit, or wiring. You can't check it with a mulitmeter because Deere has a safety function in place that won't allow voltage to the connector.
The seat switch can fail or break. You need to bridge the plug with a split pin or small wire. If the wires were broken inside the plastic coating at the clutch you would still get a quick flash of your overhead speed alarms. Sounds definitely like the seat switch / plug. Has someone been cleaning down there?? Toolbox? Esky? Kids?
How long does your unload auger run for??