The truck will have a ground strap going to the frame from the battery and to the frame from under the foot starter linkage on the bellhousing, you may want to check and make sure they are making contact. As Bill said you usually find two thin insulator plates inside the switch. It is a copper lug to a copper button essentially as a connection. It arcs and is destroyed over time maybe it has something wrong with it internally. it is really simple just the 2-screws and the battery connection hold it on.
