Rear window wiper

Just recently, the rear window wiper has started jumping, rising about 3or4 inches. Sometimes it will complete the cycle of wiping the rear window, but mostly its just bouncing around back there making all kind of noise.
The dash switch to activate the rear wiper is not on, when I turn it on the wiper wil cycle normaly, when I turn it off it's starts jumping around again.
Also, when I put it in park, it stops. Put in R, N or D and it starts jumping again.

Any one heard of such a problem?

Problem resolved..battery needed water.
Don't have a clue as to the relation between low water level in the battery and rear wiper but after filling waterlevel in battery the wiper problem has never occured again.

interesting, thanks for the update.