I have a 97, 3.0, 2 wheel drive Ranger. Every time the wiper blades turn on the engine looses power, the lights start flashing, and the truck starts jerking, and the speedometer fluctuates all over. I've replaced the alternator and checked it it is good to go. The truck only does this when the wiper blades are turned on. As soon as I shut them off the truck goes back to normal. What could cause this problem? A wiring short?

 

I would check for a loose or bad ground at the wiper motor.
Maybe the multi function switch