On my '14 Seneca that switch controls the overhead ceiling lights closest to the cab. It has 3 positions, "up" is lights on continuously, "down" makes the lights controlled by the entry door switch, "center" shuts the lights off entirely regardless of door position.
If yours doesn't do anything regardless of position you have some sort of problem. Mine has a relay in the circuit that is controlled by the magnetic door switch, if there is a problem with that it would cause the lights to not work at all.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST