Abstract: The pump drive mechanism is driven by a wiper drive motor having a drive pin adapted to revolve in a circular path, while driving the wipers, and which is displaced along a generally radially directed path outwardly of the circular path, at a prescribed point thereon, when the wipers are moved to the park position. The mechanism has a rotatable drive cam, which is connected, via a cam follower, to reciprocate the piston rod of the pump. To facilitate installation, the drive cam carries a drive pin engaging portion designed to automatically engage the drive pin, after the first actuation of the wiper motor, regardless of the initial relative positions of the drive pin and drive cam. As a result, the necessity for positioning the drive cam with respect to the drive pin in a prescribed position during installation is eliminated.