Your dome light and the battery draining may not be related issues
I suggest you debug them as two unique issues
Dome light
Your switch is still a switch no matter what direction current flows
The switch physically has to be oriented so that off is aligned with the housing the wires on the switch can be either way and the circuit will work correctly.
Battery drain
Start at batter and follow the current,
check the voltage regulator, the cut out coil could be sticking shut
Engine off check for voltage at the generator, everything should be 0V at generator if not that’s where your battery is draining.
-s
