Have you taken the motor off and run it directly from the battery?
If it works properly then, it will probably mean that the wiper mechanisms to which the wipers are attached need to be dismantled and regreased.
That was the case with my 1984 240. I could not do it without taking the dashboard out.
If you take that path, this US LHD Cleanflametrap illustrated guide helped me a lot:
http://www.cleanflametrap.com/dash.html
David.