[FWB]

my guess would be the incorrect switch is being used. wiper motors come in two basic flavors. with dynamic parking and without dynamic parking.
does the motor have a more than two wires, if so you will need a switch that has a hot on the off position. this hot wire will feed the parking mechanism inside the motor. this hot feeds a clock switch inside that will interrupt the circuit at a certain position. without it they just turn off wherever they are.
you cannot expect a toggle switch to work in the system, it must be a wiper switch.