When you turn off the switch, there is a second switch in the wiper motor that continues to run the wipers until they are parked. If the window where that switch does not keep the wipers running is too narrow, the momentum of the motor turning will carry the wipers past the end of that window—they will continue until they are not-quite-parked. But not-quite-parked, in an absolute sense, is not parked at all, and that switch will reestablish contact, running the wipers to try to get them to the parked position.
Once there, the overrun happens again, and the cycle repeats.
Beyond that, if that switch gets stuck it will keep running the wipers, parked or not.
Sometimes the wiper motor can be easily accessed, opened up, and that switch can be serviced and properly adjusted. Sometimes the motor needs to be replaced.
Find the connector that runs the wipers, and disconnect it while the wipers are running. The blades will stop where they are.
intermittently, and briefly, reconnect the wipers trying to get them to move only 5 or 10 degrees at a time. This will avoid the motor get up a head of steam. If the wipers stop responding to this once they are parked, the window that the switch considers "parked" is too narrow. If they just keep responding to the application of power, even when they are really in the parked condition, then the switch has arced shut.
1995 BMW 5...
on Nov 08, 2013