Do you hear a whining like the pump is trying to send fluid back there? I'm pretty sure it takes off the main washer fluid reservoir in the passenger fender, as all the times I've ever torn apart the interior I never found a separate reservoir for the rear washer... Otherwise I'd suggest just emptying it.
My initial guess? I'd say you either have a switch that is stuck (on the column stalk) or something funny going on with the wiring for said switch.
