1) what age is the vehicle?
might need a new replacement part if it is an older vehicle
2) do they judder accros the screen?
if they judder accross the screen then take the motor off, disconnect the plug, remove the screws in the plate, take the plate off, check the contacts inside and clean the grease off then with a bit of emmery or wet'n'dry paper, rub the surface down, replace all parts in reverse order and try that,
hope it works
if not it will require a new motor