Try a new pump from a MK3, you'll need to chop the plug off and splice it to your loom. Main advantage is the non-return valve makes the rear wipers instant on rather than having to wat 10 minutes for it to start spraying

Late MK2s also had the non-return valve, but a MK3 one is more likly to work and will be in better nick generally

While you're there, grab the black plastic hose from the MK3, looks much tidyer than the horrible yellow crusty stuff on the MK2. Also the MK3 washer jets will fit, so nick them too in case yours are blocked
