Windshield Wiper/washer Motor problems of the 2006 Ford Ranger

I purchased a 2006 Ford Ranger brand new in may of 2006. Soon after at probably 5000 or so miles my windshield wipers would come on for a few seconds and go off without me having turned them on. Then when it would rain hard and I had to use the wipers in high speed mode they would not shut off when I turned them off. Then also sometimes they will not come on at all. The other day it started pouring down rain and they would not come on and I nearly had a wreck. I had to pull over and wait for the rain to stop. I have taken the truck into a dealership and they said they could not find any problems. But the problem is only occasional but is getting worse. My vehicle is now at 47000 miles and out of factory warranty. The problem is still ongoing and getting worse.