GMC87,
There is only one true solution. Add an additional cut off switch. The GM engineers were very "smart" in their electrical wiring by having power to the wiper motor all the time. To keep the wipers from moving when you start your trucks, turn the key quickly to start the truck. I had a similiar problem while driving. The wipers would almost always come on at a stop light. I replaced the wiper motor and the problem went away. I still have to put up with the movement at start up. I hate it espcially after I washed and cleaned the truck.
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Gerardo
1983 Custom Truck
TX Plates: "BAD BOW"
http://www.geocities.com/abetterchemist/