Dear Paul,

I don't know the arrangement in the W 123 T but in the W124T
ther is a huge bundle with 12 elec. wires going along the left hinge of the rear door. Aging and opening and closing the rear door can, after 10, 15, 20 years of use, cause a lot of spooky electric problems.
On the W 124 T, the rear window washerpump is controlled by the rear window wiper motor. So if this is the same on a W 123,
don't forget to check this wiring and don't be surprised to find some broken wires inthere!
Just before the l/h hinge, under the plastic cover (easy to remove), there is a large connector with 24 screws connecting the hatch wire harness to the car's harness. When you check this out do the right side too! Often the tube of the washer fluid got broken or dammaged as do the wiring for the rear window heating.
Good luck,

Danny