Reversing sensors get their power from the reversing lights which require that the reversing lights be on which in turn need the reverse selected switch to work.
By "selected" you mean I should be in reverse?
And to all of you: Yes - when I shift to R, I hear two beeps. The 2nd one is very staccato in comparison, which I'm sure isn't anything to do with it. My reverse lights are definetly on, but the one thing I can tell you that I do notice is that they aren't nearly as bright as reverse lights I've had in the past - however they light the areas visible in my rear view mirrors very well.
And everything is stock standard. Is it possible that they have been deactivated in any way, or perhaps damaged somehow?