On windscreen wiper problems on 90's Merc Hymers

If the motor whirs and turns but the wipers don't, or they move erratically or incorrectly then that is usually faulty or loose linkages - you need to get your head in under the front access hatch and get a good look - it's almost impossible! Mirrors are an option....the wiper motor is mounted near the passenger wiper up under the sill directly below the edge of the windscreen. It is a rotary motor. This drives a concentric cam, which is linked to arms connected to small arms on the back of each wiper arm, left and right. It's a lot easier to look at than explain in words! Somewhere in here you will find one of the joints/linkages is loose or slipping. Another good check is can you actually hear the motor running when the wipers are not! If you find the problem, you have to make sure the motor is in it's rest position - so cycle the wiper switch so you know the motor is in the correct off position. Then you position the wipers where they should be across the bottom of the screen, and then you tighten all the joints and linkages back up. It's like Meccano basically - if you are old enough for that!