99 f350 wipers cut truck off

[f350club]

well here is my first post.i have a 99 f350 powerstroke that i bought used.when you turn the wipers on intermittent it will cut the truck off if idleing,or it it act like you turned the switch off if you are driving it.does not do it on high,just the intermittent settings.i know it not something getting wet in the rain because ive tried it when its not raining.took it to a ford mechanic and they cant find anything wrong.thanks