The windscreen wipers fitted to the cab of a driving vehicle varied, sometimes there was just one for the driver, sometimes there was an additional one fitted on the non-drivers side. On a cab with three windsreens there was never a wiper fitted to the center screen - except in some non-passenger cases such as route learning vehicles.
The wiper motor was air operated, and often had a handle fitted for manual operation. Windscreen washers were aften fitted, also air operated, although there were often disconnected in later years. The wiper blades were pretty standard, differing in length depending on the size of the windscreen. Most Derby Lightweight vehicles (those built after the West Cumberland batch) had a blade that extended in both directions to cleat the screen above and below the motor.
There were several different types of windscreen wiper motors used. Each type of motor would be available in different versions depending on the angle of the sweep of the wiper arm.
One popular type was by Trico - the FPK 375 along with the F1430-1 control valve, as seen in the exploded diagram.
Thanks to Trevor Daw (DMU Group (West Midlands)) for the loan of the brochure.