When I turn on the switch to turn on the rear wiper it makes this rrrrrrrrrrr sound and doesn't move. So I get out and go to the back of the vehicle and give it a gentle shove and it starts to work for about 4 wipes then stops and the sound rrrrrrrrrrr happens again.(so the motor isn't spent...right? Cuz it's trying to work) What the? any ideas on what is going on? With all the salt, sand and snow it's a pain in the @ss not to be able to see out the back. Thanks all!