2002 Mercury Villager Washer Fluid Warning Light

The warning light for washer fluid is on. Have filled reservoir for washer - still on. It seems that the washer level for the rear window wiper may be empty, but I do not know how to fill. Is the rear window washer pull fluid from the same place as the front? If not, how do you fill the rear fluid? Is it most likely a sensor? If so, how do you change that?

Thanks

Hank