2005 jeep grand cherokee rear wiper not working right

2005 jeep grand cherokee rear wiper not working right I have a 2005 Grand Cherokee. Something is up with the rear wiper; it works just fine if the switch for the rear wiper is already in the "on" position when I start the truck. If the switch is "off" and I try to use the rear wiper after the truck is started...nothing. Doesn't make any difference what speed it is at . It also seems to only work that way if it is warm out, when it is cold it wont work at all.