good to find another sunny coaster on here.
from memory the washers are driven by a small gear or cam setup that runs off the wiper motor.
they have a rubber diaphram that acts as the pump .
maybe it has perished or split. i dont know how you would go about getting a new one, maybe try Rare Spares.
on my XB i replaced the original washers with an aftermaket washer bottle and electric pump mounted in the boot.
i removed the pump section from the wiper motor and fitted a top plate from an XC to tidy it up.