You'll probably have to ditch the light, I think, or persist with the switch in one position only, or send it back to the guy you bought it from and ask for a warranty claim (good luck on that one!!!).
I had a Senso Plus like yours and was extremely happy with it for a year or more of commuting and randonnees... then the slider switch went on it. And the switch is almost unserviceable without busting the plastic bits and pieces, which leaves you with no light. The same applies to the ordinary flick-switched Ovalplus, which also went cactus.
I am afraid these issues are what eventually turned me away from the Ovalplus to the E6, which has a magnetic reed switch and the electronics are pretty well sealed. The ingress of water is obviously a design flaw for the Ovalplus... as the North American importer even admits, there are drain holes there to let the water out, and not to mount the lights upside down because of this. It's a shame, because the optics produce a beam that is one of the best, in my opinion, for spread, intensity and penetration.