Dim Rince Only Light
You are getting a slight feed through of voltage through the selector switch which is just enough to cause the light to light slightly. You can try one of several things to correct this or leave it alone as it won't hurt a thing, 1 paint the bulb black so you will only see it when it is on fully, 2 replace the selector switch or try to clean the dirty contacts by flushing contact cleaner through the switch, 3 Try connecting a 100 watt light bulb to the wires that go to the RO light, doing this might burn the dirt off the sel sw contact that is allowing voltage to bleed through to the light, 4 Add a voltage dropping resistor in series with the RO light.
You could also try replacing the light package itself as the RO light could have become too sensitive to voltage [ although I think this is pretty unlikley, but I have seen stranger things ]